I was planning to feed the remaining cold taps and loo cisterns etc in
the rest of the house as well, but is there any benefit in doing so, or
is it just going to cost more in silophos balls?

Well, toilets use a lot of water, and hence phosphate. Personally, I would
dose them though. My toilet only stays clean for a week or so due to
scaling.

What about the (hardly used) outside tap?

Definitely not.

Christian.