Nigel, the seller/buyer guides are here:
https://undeveloped.com/help/seller-guide
The commission is 9% + VAT, but unlike Sedo, there's no minimum commission fee. I'd messaged them about that and they said no minimum fee at the moment, but it may be introduced in future.
If a domain is taken at BIN, when payment is received you get this message:
I tell them the domain is held on my own Nominet registrar TAG and tell them what my tag is. I assumed by transfer that they meant transfer to them to hold in escrow, like it says on their sellers page:
Instead, they give you the buyers email address like they do on Sedo, as they send this message next:
Lat them know the transfer has been sent, and when the transfer is accepted, let them know the buyer has accepted it and that it's now under their control, they presumably check the Who-Is and/or ask the buyer to confirm that, then you get paid.