Dear Linzoid
Sis is right - Relate's fees are very flexible. I was there just over three years ago (by myself, as vile ex-husband refused to come with me) but it cost me about £15 I think, as I was on a very low income. I know couples do cost more, but again, it's very much dependent on your income.
I am so sorry to hear you and your partner are having problems - but I know four sets of friends who have done couple counselling, of which one couple have split and the other three are still together. I gather that there really is a very good chance of it helping, a lot, IF both partners are willing to go and genuinely want to make the effort, AND/OR if one partner hasn't already made their mind up they definitely want to leave and refuses to reconsider no matter what, which was the case with the couple who did split after the counselling. I have also heard, reliably or not I don't know, that rows, however fierce, are easier to work with than silence and sulks. One of the other friends was convinced her relationship of 20 years was irreversibly smashed - man having affair for months and lying through his teeth etc - until she was so hurt and angry she could scarcely be in the same room with him without screaming, but over the last year or so they've gradually got it sorted, and are on better terms of understanding each other than they ever were.
At very least, it does give you reliable support even if the worst happens. My worst felt pretty awful at the time (after saying he really wanted to have children with me my husband of 8 years started seeing another woman a week after I had a miscarriage and ran off with the strumpet five months later...) and it was so important just to know I had that hour of completely calm and neutral support just for me every week.
I do hope things improve for you. All the very very best.