Well that link is an office chair. I hope it doesn't taste like an office chair.
I did some digging and I think the kit you bought (if I had to make a guess) is this one:
http://www.argos.co.uk/static/Search/searchTerms/HOME+BREW.htm#pdpFullProductInformation If there is one thing I have learned here it is that it is crucial to get kits that aren't old. Your yeast could be old or the LME (liquid malt extract) could be bad. You mentioned a can and heating it, that sounds like LME. The directions are to add water and sugar and you're done. Just looking at the kit makes me suspect that it is worth anything. I would sit on that beer for a while and see if it mellows. I think you're in the UK so I found a place that has kits. It seems like this place is more reputable than the one you found at Argos.
http://www.brewuk.co.uk/store/beerkits/banks-original.html Look around that site and see what you find. You can create your own recipe as an extract brewer too, do not feel limited to kits. I just put together my recipe for my fourth brew based on information I found here on this site under recipes.