I assume you have already contacted Clive Stratton who has been the UK person selling Nauticats for a very long time, new and brokerage – http://www.strattonmarine.com/ . He is extremely helpful and knowledgeable and does not do a hard sell because he knows people need to take their time. When I was looking at buying a secondhand Nauticat via him the owner was away for a period. I asked for a test sail of an identical boat and within days Clive had fixed it up for me. I in turn am on Clive’s list of people who will do test sails – unfortunately for you I have a 321.
I appreciate you are looking for a longer session on a 331 but you never know….
Hope you are successful