Decided on Stockholm in the end.
We were going to use RyanAir for the (£79 return) but the airport they fly to is quite far outside Stockholm, so the extra cost of getting into the city would probably add up to the same as flying with BA from Heathrow (£100).
Also, as its 3 months in advance, loads of nice hotels have some great offers on. Either going to go to one of the
Nordic Hotels (Light of Sea) or the Rival Hotel.
Both work out at about £90 each (two sharing) for two nights.