Top place candidates for consideration have been:
Amsterdam
Brussels
London
(My suggestion and choice, without considering other factors, was Amsterdam. The other two suggestions, Brussels and London, were from Sven, who (correct me if I'm wrong) favoured Brussels.
Looking over Skype logs, London seems to have more cons than pros, and Brussels seems to have more pros than Amsterdam.
For this reason I suggest we choose Brussels as the city to host the ImmInst European Conference.
Regarding dates, I suggest we look at the dates in the appropriate time of year, remove the dates of events with which we don't want to clash, then offer up the alternatives to the membership as a poll, then make a decision based on poll results.