Thanks for that link grumpyfrumpy, it was very helpful indeed.
Well, I've just completed a very basic 'test' using the BBC link GF gave. I made a table with 3 columns, one for each of the big 3, and then went down the BBC summary of the issues & related policies, putting a mark in the column of the party that had the policy closest to my own feelings on the matter. Labour came out on top, but it was a very closely run race (although the blues were still trailing behind). I then did as sophable suggested and chose my 3 'top issues' (for me, Health, Education & Crime). Labour won 2 out of 3 and LibDems won the other.
I guess that has confirmed my left-wing tendencies and I feel much more confident about my vote now, so thank you all very much