Is the UK EU membership bill paid in GBP or EUR?
If it is paid in euros, is this not inefficient because eventual credits (eg development funds) will need to be converted back into sterling, so a conversion loss is experienced twice?
Every payment into the EU coffers are made in EUR, obviously. Same rule applies as when any payment into HM coffers is due. OBR already took that into account calculating EU contributions due from UK. Please see here and here
Polish government, for example, takes advantage of that by taking advantage of - and/or "influencing" it further with other instruments available - the conversion rate when they are due.
Yes, the conversion is done both ways. But obviously, not necessarily at a loss
It depends. A lot of the money "paid" to the EU doesn't actually go to the EU. It is spent in the UK, on UK projects or as grants/subsidies for things in the UK like farming and culture. In that case the money is simply transferred directly from the UK government, in GBP.
In fact, there is no "membership bill", only a commitment to fund things in the EU budget. If those things can be funded in GBP it makes sense to do so.
Data from the Office for National Statistics: https://visual.ons.gov.uk/the-uk-contribution-to-the-eu-budget/