Hi Tom,
I had an Endorectal Coil scan at Sutton Branch of The Royal Marsden in 2014 from which it was perceived I had a small tumour in my Prostate. I was then referred to UCLH where I had further scans before having HIFU. I mentioned the Endorectal scan to my consultant at UCLH and he did not rate this type of scan compared to the MRI scans that they did which he said were the best in Europe. So a difference of opinion between two leading hospitals.
As to the experience of the E scan, it was not painful but was a bit like the reverse of excreting a normal sausagelike motion, so when inserted you felt you wanted to pass it out. No great problem for the time involved but you must not try to expel it.