Get the Shires/Westropp Brushing / Knee boots - a doddle to fit and they definitely won't drop down/trip your horse up - PLUS you can canter safely in them!
I also do use the leather/rugging ones (in fact my horse did fall wearing them) and thank God he wasn't marked - (keeps fingers crossed). But IMO they are harder to fit and a bit fiddly in cold weather - I have very poor circulation and my hands go completely dead) hence I now use the Shires ones.
get porter knee boots they are rather crudley made but wouldnt put anything else on my horse they are fantastic i really do rate them. they are only 24 quid a pair but they are brill. i wear them out hunting every time and would never go hunting with out them.
Dever Saddlery do some great leather ones under the Ascot brand. I use them on my boy.
Their website is www.deversaddlery.co.uk - click on Ascot bridlework on the left hand side and then select 'other items'. Knee boots are at the top.
thanks for your recommendations, i have bids on a couple of pairs on ebay now, 2 different sorts, an all in one knee and brushing boot and a leather pair so fingers crossed i will win one! don't mind if i get both actually, will give me a chance to see which ones are more suitable for my ned.