Right then,

4 days 4 curries.
The only difference between day 1 and the rest was i used a yellow pepper day 1 and a red pepper for the other 3 days.
2nd day.
Same curry same ingredients but with fresh frozen gravy.(same batch) Result = not as good
3rd day.
Same curry same ingredients but with mature frozen gravy. Result = as good as day 1.
4th day.
Same curry but with veg oil and mature frozen gravy. Result = as good as day 1.
It Definately made a difference!Maybe the difference was that this gravy hadn't been kept in the fridge and was stored above room temp for 2-3 days. (it Was a hot few days)
How hot does a BIR kitchen get with the gravy left out all day and possibly all night, even 2 days?
So from now on my gravy will be sitting on the worktop for as long as i dare before freezing. ;D
Cheers, Frank.

p.s. there are two other possibilities. yellow pepper (doubt it) and the sloshing about for 500 miles. But I still think mature gravy sounds the most likely conclusion to me.
