Hi ridingmilly, welcome to Western! Not knowing anything about your saddle or whether it's got leather or synthetic fenders, I can only give you details of various UK western tack vendors and general info.
To find out more about Western in the UK, look at:
www.thewesternshop.com - forum with general info and links to most UK Western organisations, horse breeders, retailers, trainers etc.
www.wes-uk.com - the Western Equestrian Society
www.aqha.uk.com - AQHA's UK affiliate association
Tack:
http://www.pullmanwesternstoreuk.com/ - Pullman Western Store in UK stocking Pullman, Continental and Montana saddlery, and I know they've dealt with a few needs for short fenders.
http://www.bootsandsaddlescompany.co.uk/ - Cattleman, Westwood and Garland saddles
http://www.ride-western.de/onlineshop/index.php - March Horse N Tack
http://www.westernscene-uk/com - Another tack shop
There's also Hooves Equestrian, Westways Saddlery the Western Saddler, so lots of people to look up to resolve your fender size issue.
Good luck!