I'm not sure it's strictly true that there aren't many people in your position - there are certainly a lot of people not already on the housing market who have no chain due to the difficulty getting a mortgage in the last few years. Obviously a good deposit is what you need, this is the main stumbling block for a lot of people, people who simply don't have £25-30k hanging around.
I think, as an excuse, hers is quite viable (I used to work for developers and often they were at the top of the chain and had to add more money at the bottom).
I guess it all depends on whether you're happy to walk away. If you're happy with your offer and you'll find something else then I wouldn't even think about meeting her half way.