Nige321,
I'm keeping an open mind. Despite it seeming very difficult to fly a R/C aircraft in such a way at night, the best evidence is the word of the crew.
I certainly think it's the most likely scenario, because of the reported size of the other aircraft. I don't think for one moment that there are little green men chasing police helicopters over the UK skies.
The only other possibility I can imagine is that it was a group of helium filled party foil balloons reflecting lights in the darkness. I have seen this myself a few times. They appear to be moving faster than the windspeed blowing them along and even appear to manoeuvre as the aircraft flies past them. I'm not in a position to say if this could have been the case here.