[Kasana]

I am undecided between Soay/Shetland/Hebridean  
Only looking for 3 or 4 just to keep the horse grazing tidy - they have to be light on the ground as some areas get a bit boggy due to high % clay - have kept Jacobs in the past.
Thanks in advance 

[Fleecewife]

Soay are certainly light on the ground and have excellent feet so should tolerate your wet ground better than white Shetlands.  Hebs and coloured Shetlands will come somewhere in the middle, but all have good feet.
You need to consider also ease of management, companionability, horns which could damage equines in a fight, purchase cost and ease of sourcing.

I think I would plump for coloured Shetland wethers as they are easy to source, inexpensive to buy, readily available, easy to manage, no horns, can be roo'd if you don't know how to shear and would be calm with horses.

[Daisys Mum]

I would agree with Shetlands definitely less flighty and good round horses, nimble enough to get out from underfoot (hoof) but without the horns to inflict damage either to animals or rugs. I find mine also are good for tidying up the rougher bits of the fields.