Thanks for the comments.
Yes, thanks, there are various gasket materials on ebay here too (uk), but most of those that come up for high temp / exhaust appear to be a mesh material core with perforated surface material so I'm uneasy about how that will work with the water. I imagine I can find something somewhere but don't know which of the many materials is most appropriate (and on ebay particularly the descriptions aren't all that helpful).
Yes, the water is for a closed coolant system so it's under typical coolant system pressure.
A liquid gasket goo thing might indeed be an option. Certainly in the short term (since we're aiming to see if it'll run again on Wednesday) I've made an aluminium plate which I'll use with gasket sealant stuff to sit sandwiched between the two (and which will block the water channel). I've additionally made a second aluminium plate to sit on the silencer side (ie not in contact with the water) of my first plate and on this one I've cut a small channel out of the bottom - obviously this will leak some exhaust but I'm initially more concerned about water leak and hopefully this will allow me to see if my gasket isn't sealing the water channel fully. I'll not seal this second plate so I should also be able to take the silencer off to look and see if I've any water leaking.
I was just after any suggestions for if there was something more 'official', but who knows, if my aluminium plate with gasket goo seems to work then maybe I'll end up with this as a long term solution.