looking for advice from any m/f couples that have enjoyed adult fun in tidal mud ie location in uk clothing clean up and and other useful information as it is something we wish to try

Tidal mud? Be wary of broken shellfish shells - those buggers can be sharp.

I don't know your location but I know of two spots that are almost completely secluded for undisturbed fun.
One is on the Humber estuary, about a mile upstream of Brough on the north bank follow the levee footpath until you get to a creek, walk toward it, past some reeds, there's an area of very deep soft mud in this creek and you can wash off in the river itself.
The other is an area of mudflats on the Severn estuary between Portishead and the Avonmouth docks, follow a small footpath along the levee until you get to a creek which borders port land, follow that and you have an area of even deeper mud. Washing off is best done in the creek there though you will have mud up to your knees or thighs getting back out, so pack some trackies!
The best area overall is the Huntspill sluice also on the Bristol channel which has a lake of bottomless mud surrounded by banks of soft clay, I don't think there is a better place to have sex in the mud in the UK, you can do it in standing position neck deep and washing off is easy just use the river behind the dam. This is dogwalkers territory though so best done under cover of darkness. One other area I have scouted but not used yet is a meander on the river Wye close to lancaut Fort and the wintours leap. Another huge mud lake in the bend of the river but tricky to access.
Good luck!