I have a pair of connected EI 151 smoke alarms, must have gone in when
the house went up 10 years ago.

Then they should be replaced entirely, not just the batteries. The sensors
degrade over time and should be replaced about every seven years. There is
often an expiry date on the casing. Batteries should be replaced at
manufacturer's recommended intervals, which are usually 1 year for alkaline
and 5 years for lithium cells.

Coincidence or cause for concern? The boiler wasn't going & it has no
pilot light, so no gas burning anywhere. All 3 going at once, what are
the chances of that?

Quite high, actually. What is worrying is that you waited for the low
battery signal. The batteries should be replaced on a schedule and should
never get low.

Christian.