Hi smudger
If you are just forming a partnership, not a limited liability partnership, then essentially you are just two or more self employed people trading. You need a deed of partnership drawn up detailing capital introduced, profit split, working arrangements etc but from an accounts point of view theres really not much more to do. Is this £300 for the deed, or for registering a new company or just for the privilage of becoming a partnership with your accountant, because if its the latter I would question it.
On paying in advance I wouldnt, some accountants take regular payments could be monthly dd could be quarterly chq whatever to pay for the years services as you go. A lot invoice after the accounts. I think it depends on your relationship with them but they may try to "encourage" you to pay upfront - why not we'd all like to be paid first wouldnt we?