Please forgive me if I unintentionally break any rules - I’m new here. I have a NAT01/NAPST bought second hand a number of years ago, and promptly sent to Salisbury for a full service. As a source I rate it very highly - a string quartet broadcast live from Salford appears to be at the end of my room. The tuner gave impeccable service until late 2022, when it started occasionally flipping to mono. I’m 40 miles north of the Rowridge transmitter and have a four element aerial in the loft. Possible reasons include 1. the signal is marginal for a NAT01; 2. Trees in the wood to the south of me have grown up; 3. The run of coax between aerial and tuner is aging; 4. There’s an intermittent fault in the tuner itself. If it’s a signal problem I have a Ron Smith 17 element Galaxy in store that I could deploy, and/or I could replace the cabling. If it’s a tuner problem I guess my love affair with one of Naim’s best products is coming to an end. Any suggestions please?
Welcome Charlie. A couple of questions, if I may;
How long ago was the NAT01 serviced?
Have you been able to put a signal strength meter on the feed to the NAT01?
Welcome Charlie, 40 miles from the 250kW Rowridge transmitter and a 4 element on the roof !!! believe me you are not short of signal …… unless you have a damaged or disconnected aerial or coax downlead.
Rowridge 40 miles out should give stereo with a jerry rigged dipole in your room, if it doesn’t I suspect it’s the NAT-01.
Thanks Richard. It went down to Naim in July 2014. I confess I haven’t put a meter on the feed, and I should - is there a cheap but sufficiently accurate meter you could recommend? I did try looking for one on the web, but rapidly lost the will to surf . . .
I should have mentioned that I also have a Creek CAS3140 in a different part of the house, and that doesn’t exhibit the same problem.
Not sure, When I’ve been curious I’ve tended to use an old FT5500 tuner, which supposedly had a fairly decent meter. A local aerial installer will probably have one. Or perhaps another member can advise.
Don’t let it come to this!! The NAT01 is Naim’s best ever source component. Even if it needs a service, it IS worth it.
As Richard says, a local aerial installer should definitely have a signal strength meter. If you couldn’t borrow one, it may be worth the call-out fee just to rule out aerial and cable.
Is the second tuner being fed from the same aerial? If so, how is the signal split?
If you don’t have access to a meter, two options. Either run a new cable, using decent coax, from your loft to the NAT01, in a single run, paying careful attention to ensuring that the core and outer braid is all clean and not touching.
The other option is to create a new short length of coax from the aerial and try the tuner up in the loft. It’s not the best idea, particularly with two boxes, but it may be an easier alternate.
Good luck with finding an FM aerial specialist; you need someone who understands radio aerials, rather than a tv or sat rigger.
Thanks everyone for the welcome and the help. To answer the various questions: sound-hound the Creek is running off a separate aerial, which, to Mike-B’s point, is a simple dipole up at the front of the roof (but inside). The NAT01 is fed direct from the 4 element aerial without, to my knowledge, anything in the way, but it’s a longish cable run (put in when we built that part of the house). The challenge is that the flip to mono is occasional and it only started 18 months ago - this entire weekend the tuner has delivered good stereo. Its decision to flip, of course, will be at times when I’m trying to record live broadcasts. I have a couple of museum pieces in the shape of a Nakamichi LX-5 for cassettes and a Sony MZ-RH1 for CD-quality digital. Two things I’ll try - the first is to run a new length of WF100 from the aerial, through the loft hatch, to the tuner. The second is to get hold of some form of signal strength meter. I just wondered if anyone has come across this problem before, and whether it could be an aging component in the NAT, which is playing up intermittently. The trouble is the £450 is a lot of money to find out it isn’t - and that was the last quote from Salisbury, although I’ve since heard that they won’t service NAT01s any more.
Certainly try to rule out any aerial issues first. Tuners are not my forte, but i believe there are some FETs inside the tuner head that can cause loss of stereo. I’m not too sure if intermittent stereo is one of their fault symptoms though. 
Regards
Neil.
Looking at the owners manual it does say that aerial should be horizontally polarised not vertical and in the case of a signal becoming weak the tuner defaults to mono to save reception.
No preamps or boosters recommended just good quality coax and horizontal polarisation.
http://ukhhsoc.torrens.org/makers/Naim/Manuals/man_tuners.pdf
Intermittent problems are often the trickiest. Skeptikal thank you for the manual, which I never had - I’ve downloaded that. The aerial is horizontally polarised - so I’m going to try a new run of coax.
I’ve not heard anything like this, but then I’m a long way from Salisbury so probably best for @NeilS to confirm or not.
I had a NAT-01 serviced in Salisbury a few months ago.
It would be useful for all of those wth NAT tuners, for you to clarify where this info orignated.
IIRC @graham55 has recently had a tuner serviced at HQ.
Charlie how does the flip between mono and stereo occur?
A starting pint, if you have a single run, before you do anything else, is to clean all the connections and reattach the existing coax. While it may be the tuner, I’ve had lead issues before. If you are using a wall plate at the tuner end, ensure it doesn’t have any attenuation.
Is the drop into mono occurring across the frequency range? I guess you might be using R3 - does it occur on others?
Corrosion of where the coax is attached to the antenna can ruin the signal transfer. It may be that gentle winds are making and breaking the loose-ish connection there. Worth having a gander. Also, is the tuner end of the coax terminated with a PAL or BNC. If it the latter, you may have a dodgy BNC-PAL adapter.
I’ve had this - rust at the end of the first run of coax.
In the OPs case - it’s a loft aerial, but similar applies.
Have you tried the Nat where the creek is I know it’s not ideal but might show up something with the main install.
We can still service these, but if it requires adjustment of the coils it would be game over if the ferrite cores were to crack. 
Regards
Neil.