Hi sidse
You do not need to stop your HRT! Just don't! What was the reason for your U/S scan - have you had some irregular bleeding? If you are on Evorel conti then sometimes bleeding does occur - and especially if your own cycle kicks in sometimes. Also it could mean a fibroid or polyp. Post-menopausal bleeding in the absence of taking HRT is more of a worry as this is unexpected, whereas on HRT it is common.
I had the same thing - I had some slight bleeding - while on HRT but not continuous combined, so was told to stop HRT pending investigation. However I had an e-mail consultation with Dr Currie who said I didn't. I then went back to a different doc in the practice and basically told her I wasn't (stopping) and that I had consulted a specialist.
The reason the sonographer may have said everything looked fine was because she perhaps could not see any irregularities. In my case it was both thick ( approx 7.5 mm) and she could see an irregularity (which turned out to be a small fibroid) - hence the need in my case for hysteroscopy and biopsy.
From what I have read I would say it is arguable actually whether you need a biopsy for a thickened lining on HRT but I would actually be pleased because it means you are being checked out. They will be checking for hyperplasia which in a small proportion of some cases can lead to cancer, Most cases of thickened lining can just be treated with increased progestogen provided nothing is wrong. I would not be worried at all - I mean the two week rule for referral should not apply if you are on HRT so I would say the docs are being over cautious - but I would still go along with it ( apart from stopping the HRT).
Hopefully you can have it (the biopsy) as a quick in/out procedure like some of us on here - and just take pain-killers and it's over and done with very quickly. You won't get the appt before Christmas anyway and if you do just say it's inconvenient....
Good luck

Hurdity x