It's a job I did for the last 20 years. We never used to charge for transactions that fell through except in the rare cases where we were being messed about by the client. Three or four years ago we started doing so, provided we had done a significant amount of work i.e. got all the paperwork up to contract stage. The going rate in our area is £100 - £150 for abortive fees.
As well as the firm's own fees there are disbursements - third party fees. On a purchase, search fees may have been paid, on a sale there may be e.g. landlord's consent fees. Solicitors have always asked to be reimbursed for those, even when they don't charge for their own work. (These days, of course, most firms want the search fees up front from the client, because at around £200 a pop it's quite a burden on cash flow if the firm funds them for all their clients.)
[I would find the OP's figure of £550 high, even more so 2 x £550. It does depend on the area, because in the North most routine conveyancing transactions would have a solicitor's fee below £550 even if completed. In the South house prices are much higher, so the transaction value is higher, so £550 may be only a percentage of the original quote.
Your mother in law should have had a written quote at the outset and that has to say whether abortive fees are charged and on what basis. One important point is that professional firms are now taxed on Work In Progress, so to improve their cash flow very many firms do not say that they will charge abortive fees; because as soon as they do, HMRC will tax them on an extra £100,000 or so of notional turnover. She needs to check the Client Care letter she was sent, because if they don't mention abortive fees they cannot charge them.]