AFE at Kidlington got my aviation cylinder recertified , including the refill (if you pick it up - shipping full cylinders is prohibitive) for £50 inc VAT.
That's a most useful post, fitter2. I've just called them and yes that is right. Not publicised on their website
There should be other firms, as you say, but if it doesn't come up on google then what can one do? The scuba shop solution is all down to personal contacts, which is why some are OK with it. I used to get cylinders tested and refilled by a firm called Life Support Services, near Worthing. They were really nice and did it for about £30. They went bust a few years ago. They also had piles of amazingly light but old and covered in dust composite cylinders which I used to look at with much interest on my visits there but they were about £1000. Fortunately the prices of these have collapsed and my composite one from Mountain High is about 4x bigger and cost me $600.