Thanks everyone, really appreciate the replies. In the end, I did get the baffle off. Was not too difficult but after running a blade down the skin-to-baffle joint (following deriveting) I then made a tool from a strip of thin aluminium that I could slide under the baffle and lift it from each rib flange individually, rather than pulling from the end of the baffle where I'd have had a large moment. It came off ok. Obviously I did not use enough proseal first time round, just as I suspected!
If I have a leak after this, or on the next tank, I shall be using the access plate method I've now learnt about! To be honest, I can't really understand why we don't put them on as standard.
Thanks again!