If you can't resolve it with the solicitor you need to go through the firm's formal complaints procedure & then to the legal ombudsman if necessary. Look at the Law Society website, which should give you some information on the procedure.
I don't follow the cost details you quote. The first lot says pre27 April, & sthe second lot post 28 March. One would assume the first lot therefore includes the second lot but it clearly doesn't as £340 is the correct VAT for the total of £1701. You don't say what the date of the hearing was so it is not clear how much of the cost you are disputing. When you make your complaint you need to set everything out in chronological order with dates, & make it quite clear what amount you are disputing.
I think you will have to accept the part of the bill which was omitted from an earlier one. No system (or person) is perfect & it would be unreasonable for you to be able to avoid paying just because there was some delay in the invoicing.
You could ask the Law Society (I think they have a helpline which may be able to advise you) whether you were within your rights to instruct the solicitor to cease work.