On Tuesday, 30 January 2018 22:48:09 UTC, jim wrote:
I have two immersions, & what I didn't find out til I went to fit
the boost timers is that one of the exstg two immersion
switches, A, also feeds the spur? to switch B of the other
immersion....

Or you could use a Sink/Bath switch

https://www.ebay.co.uk/itm/121269923759

(more expensive ones are available if you want a recognised brand!)

This prevents both heaters being used at once,

Owain