There's several. Marine Power in Bursledon ( maybe ? ) is one. Could be French's or similar up the East Coast somewhere.
Do you know this site ? http://www.yanmarhelp.com/
As to cheaply......US like already said, if they will send them outside their designated sales area.
I have bought several parts from USA and each time I get a dollar price the same as or LESS than Sterling price and even got Yanmar (Barrus) to drop the price of the exhaust elbow on the 1GM10 because they were just too stupidly different - after all they were the SAME part!
Try putting the part number into Google and see what you get. USA suppliers are FORBIDDEN by Yanmar from selling to UK so you have to get your brother-in-law to buy them.
Last part was a complete new water pump mecahnism for the same cost that they wanted for just the shaft in the UK!
Keep on complaining and maybe one day, somebody will listen!
Don't know about cheap but as you are on the Clyde.
Sandy Wood at Troon Marine Services is a Yanmar dealer and stocks plenty parts and will get you any he doesn't have.