Resorting a bay window

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We have made an offer on a Victorian house when a large bay window has been removed and replaced with a regular PVC window with brick work around to fill the gaps. I think that it would be lovely to reinstate the bay window before doing up the room but I wondered how much it would cost/what the process would be. I assume that you would need planning permission? We know some one who makes windows so we could get a discount on the glass and frames but we would need labour and a stonemason I suppose. Would we need an architect as well?

I presume you mean they took out the whole bay and have gone back to a flat wall. If it's not in a conservation area or a listed building and you haven't had loads of other extensions you may be able to rebuild it under permitted development rather than needing to apply for planning permission. First step would be to have a check with local planning office.
Unless it's particularly complicated I doubt it would be worth getting an architect. I would look for a good contractor / stonemason preferably with some experience on older buildings. I would go for a contractor as they can co-ordinate as you will need the walls, foundations, roof, guttering and windows.

Yes. So there was a protruding bay window with a tiny bit of (also bay) wall and a bit of roof immediately above the window. That was all ripped out and walled up with a small window in the new section of wall. Thank you for your advice.