Ok, so last night I tried a few things. Firstly I tried changing the protocol from NV2 to Nstreme. This brought the ccq's right up to around 40%/40%, but for some reason the connection kept dropping every 5-10 seconds even though it was "better".
So I dropped the frequency down from 5660 to 5280, after doing a scan on the interface for utilisation. This was better, but it kept dropping. Then I changed back to NV2 and it was stable, with ccq's around 50%/50%. I then just dropped the rates on the sector right down to 6/12/18mbps only. This gives a ccq of about 60%/60% nice and stable.
It's still not as great as it was, before it suddenly changed after being fine for a while. But it's decent again now.
Is this a problem that could be a dud radio? I might go up there to do a site inspection, and maybe swap the radio. Are there any spare R52N radios about? Would be nice to swap the standard R52 in there.