Depends. If its quite a straightforward, flatish shape, you can get away with 20mm. The more complex the shape, the more the cloth will have to drape, pull, expand and contract. This makes the excess material vary quite a bit, you hac have large bits hanging, which will need relief cuts. For these shapes maybe around 50mm. If its infusion, 100mm or more.

Thanks I will get cracking on this when it's a little cooler. 

I see.

wipe the general area with acetone, then apply some glecoat, maybe in a cooler temperature. Make sure it gets in the crevices. Put a bit of cling film over it so it cures hard. On the other side, back it with some glass. Once cured, flat it back.

https://www.dropbox.com/sh/yb7qpd8ehtkt4mq/om7zUaxPE4

Do you have a photo to look at?

I had a pattern in a mould that I believed was mechanically locked. Since all was lost, I wanted to remove the pattern and do a thorough postmortem.