Under the Consumer Code for Home Builders Requirement 3.2. all housebuilderds have to inform you:
At roof, they should give you the MONTH the home will be finished and ready for you to move in.
They did, but it is very unlikely they will achieve 30 March.
My guess is they will say 30 April 2018 as this is their half-year figures date.
Minimum time for each stage from roof on to completion 6-8 weeks:
1) First fixing - 1 week
2) Floor screed - 3 days
3) Drylining and jointing - 2 weeks.
4) Second fixing all trades - 2 weeks 2 days
5) Decoration and wall tiling - 1 week
6) Finals and snagging - 1 week
I would be concerned that either the left hand doesn't know what the right is doing, or they are just plain don't know anything about how long it takes to complete each stage of building a house!
You can get compensation using the Consumer Code. The maximum is £250 for "inconvenience" which is a slam-dunk. You should also insist that the £120 registration fee is also refunded (otherwise what is the point!)
As for anything else, you will have to prove with receipts, why you are out of pocket.
This could be, rearranging a mortgage, valuation and fees. Additional legal fees, extending your rental agreement (but not for rent, you would have to pay a mortgage if the home had been ready on time), any rent you may have to pay for any term left on your rental lease.
Do please have your home professionally inspected and snagged.
(I always recommend Tim Fee of Snagging Inspections.co.uk 07768 900200)
Do not legally complete on the home unless you are 100% happy and satisfied everything is as you want it.