Was worried the XRIII senior would be far too big as its more than double the size of the Junior so did a bit more research and came up with this:
Force 4 Marine Dynamics Stingray Stealth Hydrofoil - Only £89.95 - Force 4 Chandlery
another no drill hydrofoil from Stingray - fits like a glove. Fitted it yesterday and took it out for sea trials today. Have to say first impressions are very favourable. Instant holeshot - not that was ever a real problem, but even better now. I used to be able to maintain a plane at just over 12knots - this have been reduced to about 9 - great for the Poole Harbour speed limit of 10knots. Boat is also much more comfortable at speed with much less fore and aft pitching.
So - so far highly recommended and it didn't fall off on first use! Still looks very firmly attached despite a pretty high speed dash in fairly choppy water (over 30knots) and some tight turns.