How'd I get my hooves on this piece of Whovian history? Because my best pal John was there at the very place on April 3, 1983! And here he is!
It was the first Doctor Who convention he'd ever been at and he had a wonderful time! Here's Stately (Marquis of) Bath Manor, otherwise known as Longleat House:
Here's a pretty good shot, considering John's terrible camera, of then-current Doctor Peter Davison greeting the crowd:
And yes, that convention did take place on Sunday April 3 and Monday April 4. A con on a Monday? Well, yeah: in 1983, Easter was on April 3, and the day after Easter in the UK is a Bank Holiday, which means all the banks go out on vacation and go to the rocky, stony beach, so everybody else gets a day off too!
One more interesting note: although the programme clearly says Longleat House is located in Wiltshire County in the UK, modern references on Longleat indicate it's in Somerset County. I'm guessing that the county borders shifted over the past 29 years and Somerset swallowed up Longleat. Anybody know for sure?
Anyway, to answer your burning question: no, yours little stuffed truly was not there as I'm only six (which is a very good age to be) and I didn't get to be John's pal until much, much later. But I wish I could have gone to the 1983 Doctor Who Celebration! On the other hand, I have (much more recently) been riding on a Dalek, which is more than John can say.