2002 Renault Clio 1.2 Diesel starting problem

I've had an ongoing problem with starting on the car, which began in the cold weather last winter.

Problem:

-the engine diagnostics/light (middle, amber) stays on
-the car takes many attempts to start - no strange sounds but can 'almost' start then fail multiple times
-the colder it is, the worse the problem is
-once the engine's been run a little and is warm, it starts instantly for several hours afterwards

History:

-took it to a garage and they ran diagnostics for this problem last year
-they said there was a 'funny code' but they thought it was the heater plugs and replaced the lot for £100+
-this appeared to fix problem, amber light stayed off after start up BUT
-a few weeks later, problem started again, light came back on permanently, difficult to start
-took it back when MOT was due
-They ran diagnostics for another problem (airbag light) and said NOTHING about starting problems
-when I got the car back, the amber diagnostics light was back off AND started instantly every time again...

(so it seems they were able to 'fix', at least temporarily the starting problem by merely 'resetting' it? NOTHING else was done in relation to it - only an airbag sensor replaced HOWEVER)

- a couple of months later, light came back on, starting problem back with a vengence

SO:

Any ideas what the problem is? With my limited knowledge, I'd say that whatever system is responsible for turning the heater plugs on is not turning them on, but I've no clue why. Whatever it is, clearly it can be 'reset' with no sensor replacement etc., at least temporarily.

Is there a way I can 'reset' the car/the light myself?

Or - even better - does anyone know what the problem is and what the fix is?

Any help or suggestions appreciated - I'm not keen on taking it back to the garage who clearly didn't seem to have a clue!