We've a booster pump near the hot water tank that boosts the hot water
pressure on demand. It works well as long as you realise that you have to
turn the mixer to hot first (because if you try to turn it on in the cold
position the hot water flow never overcomes the cold one). No great deal,
but I'd like to be able to do something about it.

I'm going to have to replace the bath soon (my son made a hole in it), and
I'm thinking of moving the pump under it, since I think that is a better
place for it (so that it is pumped only when the bath mixer wants it, as
opposed to any other tap in the house). I know that the correct thing to do
to fix the hot water flow problem is to replace the single pump with a twin,
but it's in good condition, and I'm a bit mean. I was wondering if it's
possible to get a flow switch for the cold water, and use that to switch on
the pump too, instead of just having the hot water flow do it.

Has anyone ever tried that? Of course, it depends on my being able to open
up the pump and find where the existing flow switch is, and wire the
additional one in parallel. Do pump manufacturers (it's a Grundfos) make it
easy for you to do that?