UPDATE: Terrain Response System fault now fixed, by replacing MAP sensor.
I was removing the throttle body as I'd suspected the gears had gone on it causing a limp mode (it was fine) and the prong on the MAP sensor fell off while removing it. That was presumably the cause of the limp mode. Went to get another one (£40 euro car parts) and cleaned out the throttle body gunk with EGR cleaner. Local independent cleared the codes for me and its now all good.
Result is much improved running, longstanding warning message gone and terrain response working.
Loads of helpful photos/guidance on the MAP here:
https://www.freel2.com/forum/topic8930-15.html
Thanks to everyone posting, which has been very helpful to an amateur like me.