If you can't get Qualcast to replace it for you http://www.argosspares.co.uk/lawnmower/qualcast/wheel/catalogue.pl?path=496909,467191:496927,638686:467482&refine=wheel&page=1

This is who I contacted support@ gardenservicecentre.co.uk

Should be a fairly decent mower for the price.The B&S engine is probably made in China instead of the USA,as they used to be up until a few years ago,but treat it well,and it should last for years (dont leave it with petrol in over winter,run it dry,remove spark plug,make sure piston is all the way down (slowly lower a pencil into the spark plug hole,and slowly pull the start cord to get the pencil to its lowest point),then put a tea spoon of the engine oil down the hole,give it a minute to drain down to the top of the piston,then slowly pull the starting cord to get the piston to its highest point.Remove pencil,replace spark plug.All done!,ready for next cutting season).
You can get lawnmower wheels cheaply of the Bay,just measure the diameter of the old one....
Also noticed it has a metal deck,which tend to rot quite quickly.You should get a small tin of Hammerite,and paint as much of the underside as possible,especially around the area where the grass is ejected into the collector,as this tends to rot first.
Do the above and the mower 'should' last for many years!.
Hope that helps someone!.

Qualcast don't actually exist as we used to know them, it's now a licensed name used on Argos own brand products so I suspect little chance of getting a replacement:(

Thanks just bought

I'm going to have to contact them too, my front wheel catches on the deck at full height.

I have this mower. A good mower but recently broke a front wheel and can't get a replacement.

still better value than a gtech