I am looking into some HAVS monitoring equipment, one rep has quoted that the monitoring device must be attached to the tool in use. BS5349 Part 2 6.1.4.1 was the information he gave. I haven't checked this, but has anyone got any experience of this? The brand that I am leaning towards is essentially a smart watch that the operative wears, rather than swapping devices between tools. 

I think you have answered your own question there.  Any HAVS measurement must be securely attached to the tool (as per the relevent BS EN), it not, say for example on the wrist, then there are many mechanisms and dampners in place (biological in this case) which are acting againt the true exposure reading.

Any such wrist mounted device would then just be an expensive and over-complicated measure of tool usage time.

Ive said it here before and I'll say it again, theres very very very few situations where actual tool measurement is necessary (as backed up by the HSE advice).  Permanent tool mounted or personally worn devices are at best a expensive supplement to a HAV specific risk assessment, they are certainly not a panacea for all your HAV issue nor prevent all future HAV claims.