There’s a “Towing Code” that the various caravaning associations and DOT put together which you can find here:
General advice for caravans in this code is even distribution of weight with 7% of the all-up caravan mass being put onto the hitch. Based on the earlier estimation of 300-350kg for a Wayfarer and gear, that would suggest you’d ‘feel’ 21-24.5kg on the hitch (so in line with mharman’s advice of 25kg). There’s an illustration on p15 of the guide on how to measure this with a set of bathroom scales and a block of wood.
The advice on getting your all-up weight is to use a public weighbridge. There’s a directory of these here by county
http://www.chrishodgetrucks.co.uk/pageweighbridge/weighbridgeshome.htm Sounds like you can expect to pay anthing from £5 to £30 for this service and some piece of mind.