Many thanks for the suggestions, all very much appreciated.
It never ceases to amaze me how much better served the US market is than the UK when it comes to shotgun accessories (over here I've not seen any pads with a choice of sizes, or indeed anyone making them on an artisanal basis).
I see from the comments that The Leatherman has shipped internationally before so he'll probably be first port of call to see if a leather slip-on will do the job, especially as his look very nicely fitted/finished.
In terms of a more permanent fix, the current stock is finished with a horn butt plate that is notched in slightly to the top of the stock (I suspect making it somewhat harder to make a wooden extension to fit?).
I've been given a recommendation for a couple of local stockers so (given there's not been a tidal wave of comments telling me I'd be insane to even think of changing anything here) am going to have a chat with them before, I suspect, letting one of them loose on another gun before deciding what to do with the Reilly if the slip-on still proves inadequate.
I just hope I've not left it too late to be ready for early partridges!
