Thanks for the input everyone. I've got lots of spare capacity on my C. drive which is 180 GB in total, 140 GB of which is available. The D recovery partition is 4.6 GB in total, but only 220 MB of which is free. I've got quite a few USB flash drives and a 60 GB external hard drive which I use for backups, so it's the D partition size that's the problem.
In the meantime I've removed some of the temp files from the D drive to separate external storage, and this has instantly made more space.
I won't really know if this is the answer though, because it's only if and when I lose my OS that I'll need the recovery partition, and only then will I find out whether it's working okay or not.
I had a feeling that there was some software that could re-create a partition on the fly as it were, without deleting anything, but don't know what this is?