Thanks to everyone for the comments, suggestions and encouragement.
In the end it all went very well, considering.
The Moviprep reminded me of those very disappointing sachets of 'lemonade' powder we used to occasionally have as kids--very artificial lemon taste with a chemical background--but I could cope. Getting through four pints of the stuff was a struggle towards the end.
This morning, everything went well. I only had gas and air1 for pain relief and didn't use that much of it. They found one polyp which has been removed and sent off for biopsy, but the nurse practitioner seemed to think it would probably be OK.
The only issue was that, in the preparatory checks, my pulse rate was all over the place, going from thirty-something to over a hundred. They did an ECG before the surgeon was prepared to carry out the procedure and I am off to see my GP, clutching a copy of the printout, later this afternoon.
[1] Once the issue with my pulse appeared with the visit to the GP mandated, they told me I couldn't have the sedation, but it really wasn't a problem.