Have you tried Argos , or a camping shop (millets)

Matalan stock a varied range of luggage at this time of year.

Samsonsite do several suitcases for those dimensions exactly. Check Samsonite stockists online for an idea of pricing, a good one is http://www.bags123.com

Matalan also have a very cheap set of suitcases or buy single at the moment which include a case with your exact dimensions, they're half price at the moment - http://www.matalan.co.uk/pages/product-view/004/042/5512/luggage

Good luck

thanks,i will check out matalan,

Have just checked Ryan Air's website and it says that hand baggage can be maximum of 55x40x20cms. That can be 55wx40hx20d or 55hx40wx20d. Even if the airline gives 55wx40hx20d which be for someone using a holdall type bag, they would allow a bag 55hx40wx20d to allow a small trolley case or backpack. I would be very surprised if they didn't.
Last trip I did was with ryan Air using hand luggage only and they didn't have a problem with my 35l rucksack which was 50-55cms high (had things in top pocket) x 40-45 cms wide (things in side pockets) and 18-20cms deep.

thanks cate_taylor, i have managed to get one from matalan which fits for only 5.50 ! but its surprising how few there are with a depth of 20cm or less,most where 23 ish.