After much discussion with the site hosts, it looks like there will be times when V-Flyer is just too busy (we're one of their biggest sites). As this is not exactly a commercial operation (!), the situation is unlikely to get better in the very short term since the next step up gets rather pricey, and the time/work required to make that step is not trivial.
If you see the 'site to busy' message, can I ask you give it a minute and try again, but please don't be tempted to press 'refresh; refresh; refresh', as this will just make the problem worse as the server tries to clear through all the requests in its queue.
Ta muchly,
Pete