Update on the toe studs:
I found that just running the 22mm pipe through a 22mm hole in the base board wasn't completely satisfactory. The Mk 2 version uses what are known
as 22mm 'slip couplings' as a bush for the assembly.
You need to be sure that you get the right kind of coupling. They need to be 'end feed' otherwise there will be incovenient rings of solder in each end, and 'slip' couplings rather than ordinary couplings which have a ridge half way down which stops the pipe running all the way through. An example is here:
https://www.toolstation.com/made4trade-end-feed-slip-coupling/p73133Shop around for these, they are a relatively uncommon plumbing part and many places will ask silly money for them.
The hole through the base board now needs to be 24mm.
In the end I used microswitches as the sensors, but reed switches would have been just as good.
Nick