I will use random numbers to show you:
Buying for 100,000 - deposit 25,000
Old mortgage 50,000 new mortage 75,000
on 27th May bridging loan for 25,000
draw down new mortage 75,000 -
Pay builder 100,000
from 27th May to 17th June you then have loan for 25,000 new mortage and old mortage.
Get proceeds from sale on 17th June and pay off loan plus interest and old mortage.
Not all lenders will allow you to hold two mortages in which case you may need to do the following:
27th May - bridging loan for 75,000 which is 50,000 for old mortgage and 25,000 for deposit and also draw down new mortage of 75,000 so you the have 100,000 to pay builder
Have you asked builder if they can wait until 17th June as originally planned?