I certainly think the weather had an effect on numbers, it probably did not stop the die hard model engineer but local members of the public wanting a day out would likely have given it a miss.
Out of interest are your exhibitor numbers also down ? it did seem that there were a lot less traction engines in the lower field than I remember from a few years back. Also a lot of the same engines on display in the tent as previous years, maybe you should invite some other modelers to display their engines to get a bit more variaty.
There is also the fact you had less of the big name traders (Chronos, RDG, Warco, Myford) which again lessens the draw and is a bit of a catch 22 situation as the traders won't come if numbers are down.
Basingstoke clubs traction engine rally and exhibition which has just started up in the last couple of years will also take some of the numbers from you as people will do one show not both. They had far more engines than you even in the past though a lot less traders though there is no entry fee to the public.
J