Apologies for bringing a thread back from the dead but I'm happy to report that I had my tank repaired by the place recommended by TheDragonLord (http://www.mg-marines.co.uk/) and it cost what I think was a reasonable ?65. I didn't have the confidence to repair such a large seal reliably.
They said that the entire back/base seal had failed and the two side panes were barely hanging on, but the bow front join still took them an hour and a half to separate from the base.
They completely removed the base, removed the old sealant from the base join, resealed the base and then added glass bracing all around the join and sealed around those too.
Given the nature of the failure, they did say it wouldn't have been due to uneven seating beneath the tank, which clears my conscience. My babies have been back in the tank for about 2 weeks now.
All in all, I'm pretty happy :) Thanks for everyone who assisted.