Engine: 2.3 F/Inj B23E/B230F: Cold start hesitation

[Clifford Pope]

For about a month now the engine has hesitated a bit when pulling away after first starting. It 's not directly related to either cold or damp weather, and only does it for the first few hundred yards if I go up the hill rather than down.
After that it runs perfectly. The symptoms feel exactly like a carburettor engine that needs a bit more choke for a few seconds.

It's the LH 2.4 system, 1991, with the earlier version that has an additional 5th injector hidden under the injector rail. I've cleaned and replaced fuses 4 and 6, and the fuel pump relay looks OK - no corrosion or apparent damp.
The car starts promptly, runs well at idle, and drives OK if I stay in 1st and don't try to accelerate too soon.

I'm looking for pointers - I presume it's some cold start setting being slow to respond. Is the 5th injector for cold starts? Could this be the engine management temperature sensor failing?

If I'd only just bought the car I'd have guessed it was normal - slight hesitation on a chilly morning before it starts to warm up - but it's never done it before, even on much colder mornings.