I have used mopay.co.uk and Envirofone. I found that i got my money quicker through
mopay.co.uk i think i got it within 4 days which was nice. That was literally the only difference. Although i did have one mate who used the site after i recommended it to him and he was told his phone was worth £30 but in fact he only ended up getting £11. I have seen that
www.mopay.co.uk is offering M&S vouchers now. If i were you i would go with them - nothing against envirofone! Great idea about fathers day btw - might visit the site myself.