Thanks Tobyjug and SIS for the replies. Some answers (where I can - I can’t test things out today because I am not there)
The digital cable is a Chord C-digital, which I think is a proper 75ohm lead. Interesting comment about whether the 272’s input is locked - I haven’t heard of this before so I will check.
I hear no clicks on toslink - the telly is connected via toslink to the 272 and I haven’t heard any clicks here.
I think the clicks are mains spike related. They are only on the digital coax input to the 272, and happen even when nothing is playing so I think its something to do with spiky type noise on the mains - as mentioned above, I had the blu ray player digital coax input selected in the 272 with nothing playing and switched on the light in the room and got a click. I have also heard the clicks when the central heating switches on. (Sorry, perhaps “click” is misleading - its more like a glitchy noise.)
I think the comment about the unitiserve mains lead is interesting. I wonder if it might be worth putting a ferrite block on the extension lead to the blu ray player and the telly to act as a low pass filter - any thoughts? (BTW 272 and 250 are plugged directly into wall sockets via power line lites. Telly & blu ray player are plugged into a cheap tesco extension block that goes to a wall socket in the same ring as the 272 & 250 sockets.)
Would the Russ Andrews distribution block be a power bar? I have a spare power line lite I could use to supply this, and put a ferrite block around it. The power bar looks well made and is reasonably priced, so I could spring for one of them and a few ferrite blocks. Any thoughts on this as a proposed solution?
Thanks again - much appreciated