The reduction in range first coincided with a friend driving my car into a brick wall (thanks EBP)
and when I checked the aerial it looked like it was broken all the way through about an inch away from the case, so not sure that would be any good to try and repair and try.
I'll try removing an aerial from a good working RX and put it on the dodgy range one and see what happens
One thing I did notice though was that there looks like there's a pad on the PCB for the non co-ax aerial version - the version that just uses a piece of normal wire - I had thought about just soldering a length of wire onto that!