I highly recommend Garden Affairs! The service is beyond comparison throughout the purchase and aftercare.
We bought our summerhouse in 2014 and elected to put it up ourselves. When we had a problem with the roof, the owner himself came out to help my husband instal the roof.
Six years later the summerhouse is as lovely as it ever was but had developed a small leak. Within a couple of hours of reading my email about the leak an installer was here fixing it and checked all the windows and doors were operating as they should ... and all free of charge