The front windows on my son's house (in the Barnsley/Sheffield area)
comprise a number of approx 7" sq sealed units in wooden frames. One
of these units has failed and is misted up and he wants to replace it.

Does anyone know a supplier of replacement sealed units in this area?

The existing sealed units appear to be held in place with wooden
beading on the exterior face and some unkown (at this point) sealant.
Any tips on how to go about removing and replacing the sealed unit?
What sealant to use, perimeter gap/spacers, etc?

TIA
Pete K