Thanks man,
I'm finding conflicting out there...
Most winch isolation solenoids seem to be 125A or 250A, even though they say they're suitable for winches up to 13,000 lb ish.
The solenoids from my 12k winch are only 100A, but as you say the duty cycle may enable higher current draw for shorter times. Even so, it surprises me that a winch rated at 540 Amp only has 100 Amp solenoids fitted.
I shall probably play safe and go for at least 250 Amp isolation solenoid (last thing you need is that failing in the heat of the moment!)
Got a few bits on order; will post some photos and details of how I get on...
J