Lof Conversion Draft plans

Ybob

Hi, i apologise now for the long post and terminolagy sorry if I have called things the wrong name- here goes - we had a structural engineer come out to give us some drawings for our loft conversion.  He quoted us £1100 for plans, calculations and submitting to building control (this did not inc building control fee) He came out popped his head in the loft took a few photos, no measurements and then measured up the rest of the house and wrote measurements on the room plan I had given him which already had room measurements on it.  He went off saying he would draw up some draft plans for us.  The drafts arrived and they were wrong.  He had put the ensuite in the wrong place not where we had asked him ( you couldn't even fit a bed in) and he had drawn the existing roof trusses completely wrong - we have a different style which he should have seen from his photos.  He has also put the new beams directly underneath our purlings which cant be moved until the beams are in (he would have known this if he had used the correct roof trusses for his drawing).  I have now told him we do not want to proceed - basically I have no confidence in him.  He has now sent us an invoice for £770, 70% of the original price for work he has done.  As of yet we have seen no correct drawings, no calculations and he definitely hasn't submitted them.  What do you think would be a reasonable payment?