Thanks, Neil… 
I had asked my Naim dealer @Cymbiosis (thinking I migt not get a quick response on here…)
They were told 3,15A (by Ian Marshall) - ??
‘My’ 250 is really a CB unit - its from 1985. It just had its clothes changed by Salisbury (in 2003), so it now thinks its a Olive unit…!
There is very little on its rear panel.
You will always get a quick response Ian, that’s why people ask us I guess!! 
Wayne had asked Ian Marshall and certainly the more modern 250s are 3.15A but with an older 250 like this and a H&F transformer, I would not doubt Neil’s advice.
So we have asked the guys in Salisbury to confer and come back to us and confirm.
KR
Peter
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Update. Here is the full rear panel of my 1985 originally CB (but now Olive) 250.
Fuse Rating - 4 Amp Anti Surge.
(apologies for missing this - a set of SA8’s and two runs of NAC A5 cover quite a lot…)
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Peter, indeed so. That’s why I called Wayne… I thought he would know - or could sort it. Didn’t expect there to be a much magick involved though…!!
My money is on @NeilS Neil S being right here… 
T3,15A is a safe recommendation (for most 250’s) - but it looks like the older 250’s like mine (with H&F transformers) should have a T4A.
Who knew Naim Fuses were such fun… 
If in doubt, ask. That’s what we do and yes with H&F and being much older too.
Important point here… Only use the fuses as supplied by Naim, as other fuses even with what appears to be the same value, tend not to have so much headroom on power up tend to go ‘phut’… as you say/found out in your o/p.
KR
Peter
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Indeed. Ask - or find out yourself. Do not assume.
My current guess is that my stock of spare ‘250’ fuses, supplied by my Naim dealer previous to Cymbiosis, was not of good quality or was just plain old - and so have blown far more easily than they should have.
I have now fitted one which is from this batch but is clearly marked T4A (which is believed to be the correct rating for my 1985 250) - and which is working.
Meanwhile, I am advised some correct T4A rated fuses are being ordered up, for me, from Naim.
Very Good Service - if you ask…
The bolt down 250 used a 3.15A @240V - different transformer again.
Yours is definitely 4A, as confirmed by the rear label.
Regards
Neil.
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Thanks Peter, the fuse in my olive late 250 hasn’t blown yet and I now know what to procure in the event it does.
And… by its Instructions…!!
(Thats what you got inside the box, in 1985… Plus a DIN-XLR lead.)
Thanks @NeilS …!!!
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You should have a spare in the pocket in the IEC socket anyway BTW, but yes insist on only the fuses Naim supply. It will save disappointment 
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Yes, but… reduce the risk and get your fuses via your Naim dealer. Please…
As @Cymbiosis Peter says - Avoid Disappointment.
I did this - but it seems either my spare fuses have aged or were plain wrong. They are/were RS fuses and do not appear to have been sourced in line with Naim’s recommendations.
A very simple explanation of my problems would be if some of my spares were actually the wrong spec. I like simple explanations…
PS. Those who are not within the UK should check carefully. The correct fuse for say 115V and the US will NOT be the same as discussed on here, for UK 250’s.
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To add to what has been written, I also received the following lists of Fuse Ratings for various Naim units, through the ages.
These have come to me from Naim (Ross Channon) via Cymbiosis (Wayne Smith).
Specific to NAP250’s -
BD 250 = T3,15A
CB 250 = T4A
Olive 250 = T5A
Classic 250 + 250 DR = T3.15A
@Richard.Dane - Trust its OK to upload these lists, as its Naim info…?
Possible addition to the Fuses FAQ maybe…?
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Further update, on my ‘saga’.
I have checked with @Darran at Class A (who dd my service on my 250, in November’23), to find out if he replaced fuses during a service.
He has now replied - “The fuse is not normally changed as part of a service. If it fails it is essential it is replaced with the correct type and value for safety. The main reason they can blow is if the unit is powered down and then on again before the caps have had time to discharge.”
So… my 1st failure, in February, was of a fuses which had been in my 250 for… many years (I cannot remember when it went in). Cause I think we can say was age/cycles - and can be considered normal.
My 2nd failure this week, is more problematic. I think its likely it was due to an incorrect or poor quality fuse. Its seems unlikely to be anything else. As @NeilS has commented above, a ‘proper’ fault would blow the fuse all the time.
Fingers crossed all is well. My latest fuse is marked T4AL but is an RS branded fuse, of unknown provenance.
Right now Rush is playing. Its all their fault anyway…
And another update…
Today received 2 off of the ‘correct’ fuses, via @Cymbiosis - thanks, Wayne… 
Very ‘different’ looking, to my old stock - see picture -
(my old stock were RS branded and were a straight wire design)
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I’ll let Wayne know you are happy when I see him on Monday 
I have emailed him already…
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Obviously Rush is to blame.
You will need those extra fuses if you ever get to Power Windows and stop listening to those collections!
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