Ideally I would use a semi-directional microphone like the k6/ME66 system from Sennheiser. Some of the Senior members of this forum have them and seem to be impressed. The main drawback in your case is that it needs to be pointed towards the sound source, either by the subject or by a sound assistant.

http://www.sennheiser.co.uk/uk/icm.nsf/root/03284

Failing that I would use a tie-clip microhone, the best you can afford. Think about a cardiod microphone rather than an onidirectional. The cardiod is a bit more likely to reject unwanted sounds. If you've got a bit of money to spare, the ew-112p G2 wireless system is worth a look.

http://www.sennheiser.co.uk/uk/icm.nsf/root/21411