@LittleBearPad
In your example the OP could pay £6k in rent over 6 months or £11k in mortgage payments. She would still only have reduced her mortgage (which is the bit that actually matters) by £1k or so. Given it would appear she's only been delayed by exchanging by 2-3 weeks the loss is negligible.
I do actually understand the point you are making – obviously, if you sell your house after a year of ownership, you will only have paid a year's interest – not the total 25 year amount of interest.
However, it's disingenuous to use this argument to imply that the OP will suffer negligible losses by continuing to rent over buying.
Because, you know, that in the real world (and this is why you own a house rather than rent) – house prices are rising consistently over the short term and over the long term. In the real world, today, in London, barring disasters, if the OP sold in a year she would get all her mortgage capital payments and interest payments back plus a handsome profit.
So, as she is in a position to buy, and the only barrier to her completion is a selfish vendor – who should be pressured to complete the chain – then she is making a genuine loss by being prevented from switching from rent to mortgage.
The rent she continues to pay is a total and absolute loss - that she has no hope of ever recovering except by seeking redress from the vendor who lied to her about their readiness to progress a sale.
And that loss is all of the rent she continues to pay as a direct result of the vendor's deceit.
If she hadn't been lied to she could have gone ahead with another house 17 weeks ago.
Of course, In this country there is no real legal remedy for her complaint – even though I believe she is morally correct – the only remedy open to her is to ask and use whatever leverage or bargaining power she has – good luck to her.
She knows the consequences – she could lose the house – her vendor now needs to think about the consequences.
And I say good luck to the OP and bad luck to all lying gits, whether they be sellers, buyers, solicitors or estate agents.