Bit of an update today. Had a quick 20min run to the supermarket and back and as usual had the burning smell on reversing up onto our drive. Decided to open the bonnet and there was smoke. Not a great deal and within 30 secs or so it seemed to stop. Most appeared to be coming up from the left around the cambelt. Will have a better look tomorrow if we get a dry hour or two. Definitely not an exhaust gas problem though.