Overcharging

[AgeingAndrew]

So we need a new solar controller. We're on the tunnel on Wednesday so it's probably too big an ask to get it fixed before we go. Anyone know of a good solar person in France (west side)? - We're heading slowly south towards North West Spain.

I can’t see how you arrived at the conclusion that you need a new controller. Just because two others report problems doesn’t mean that your unit is faulty.
More data gathering is needed first such as:
What type of battery do you have (age, gel, wet…)
What display panel is showing the alarm, what is the display panel model type
How is the solar controller configured, is it configured for the type batteries you have
What do the LEDs on the controller show.
Is the overcharge showing on the starter and leisure batteries

If it was me, I wouldn’t spend a penny before I had gathered all the facts and ascertained the problem.