Thanks everyone for all your input, your thoughts are much appreciated.
The swelling (more like a loose sack) contains a small amount of fluid that's surrounding a moveable, marble sized lump. If the lump hadn't been there, I would have thought the same as
Ellasma,
Denbenj and
Clava...an Haematoma.
NoviceNic, I don't think it could be an abcess because there's no pain and it isn't a solid swelling.
Shian89, Deacon Blue is a gelding, but the swelling is under the top of his front leg and he was gelded a number of years ago.
Spoof, I've learned something new today, I'd never heard of a shoe boil before starting this thread.
However, I don't think the swelling could be due to this, mainly because of where it's situated.
With the added description of the swelling, have you any other thoughts of what it might be?