Thanks for all the helpful replies!
Stivino, I have just phoned the company you suggested, however, they haven't got any Nylon washers. Thanks for the suggestion anyway.
imamartian, funny you should ask that; I WAS thinking of making them myself. The only problem which stands in my way is the centre hole. It needs to be exactly in the centre as I can't have any part of the washer poking out from under the fixing. I am quite good with scissors and can probably cut the outer diameter neatly. I was thinking of using a plastic wallet from WH Smith which is for putting documents in (a plastic folder)! Is there a more suitable material I should consider?
Softus, same problem as with the drill, will the revolving punch enable dead centre holes?
Jasonb, I just tried to phone them but no one is answering at the moment. I need to ask them if they have the right sizes as I don't really get all the 'M' business.
xerxes, concentric inner and outer cuts would be just the ticket, but as you said, it's too expensive and after I make the washers I currently want, I probably won't use it that much, if at all.
Thanks.
Jasonb, just called Model Fixings again; unfortunately, the biggest they do is 7mm.
Just had another idea. If washers which are wider but with the thickness and inner hole diameter I'm looking for easily available, would it be possible to get some of those and then cut the outer diameter down to 12mm? (IF these are available in the first place!)