Mine was probably in a downhill race at York (Western Australia - not UK) about 12 years ago.
After the race, gave my knee guards and body armour to my folks who headed into town for a bit whilst I mucked around on the course for a bit more. Took a 2 foot drop at the bottom of the course a bit fast and landed square on a log, stopping me in my tracks and kneeing the cable guides at the top of the frame.
Crawled myself and bike off the track and laid in the grass. Looking down, I could see a 4 inch gouge around my knee, white flesh and I think a bit of bone. After a few secs the blood started pouring out.
Folks freaked when they came back to the course to collect me and saw me in the back of the ambulance being attended to.
Can't remember how many stitches but I've got a big purple scar to this day.