Not too long ago Swansea Maritime Museum held the International Swansea Marine Model Festival. A DVD was released (in 2004) covering the second of these festivals and the museum's curator at the end of the DVD indicated there was to be a third festival, whether this happened, I don't know.
The event looked well attended and publicised, so no reason why you shouldn't gain support for an event in Wales.
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I don't think the problem is just in Wales, as a recent thread covering the subject "Long Term Future of Shows" was featured on Martin Davis's Model Boat Mayhem site.
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