Window company changing contract

Shonaec

Ordered new windows and door 26 Feb. Verbally agreed fitting mid April. Contract signed showing 6 x windows and new front door and fitting of.
Finally fitted windows this week with agreement to fit door as soon as it arrived. I understand there have been delays nationwide with composite doors. 
They cracked a pane on fitting, which is due in to them today, fitting that next week.
BUT they have invoiced for the windows already. 
I don't want to pay until the door is fitted, as per contract. (Apparently it's due end of July)
Nothing was mentioned at any point about changing the payment terms.
What are my rights? 

Keep_pedalling

So what do the payment terms actually say on your contract?

Aylesbury_Duck

What are the payment terms?  What does your contract say about when payments are due for which elements?

Shonaec

Windows and door sizes etc listed above

Aylesbury_Duck

In which case, and assuming that's the only element of the contract that relates to payment, go back to them and say that you'll pay their full invoice when the door is fitted.

Shonaec

There is a clause re payment in the terms on the back, but again says on completion of services.
When I initially offered a deposit, which they declined, they also stated payment not due til job complete.

MattMattMattUK

Shonaec said:

Ordered new windows and door 26 Feb. Verbally agreed fitting mid April. Contract signed showing 6 x windows and new front door and fitting of.
Finally fitted windows this week with agreement to fit door as soon as it arrived. I understand there have been delays nationwide with composite doors. 
They cracked a pane on fitting, which is due in to them today, fitting that next week.
BUT they have invoiced for the windows already. 
I don't want to pay until the door is fitted, as per contract. (Apparently it's due end of July)
Nothing was mentioned at any point about changing the payment terms.
What are my rights? 

Then question perhaps becomes, do you want a fight about your "rights", or do you want to be reasonable?
If I were you I would email them back and say you will pay for the windows next week (by which point the replacement pane will be in) and that you will pay for the door once that is fitted. If they are reasonable then I can not see them having a problem with that and from your position it would also be entirely reasonable. 

Shonaec

I'm just a bit frustrated that their customer service and communication is severely lacking, they've not phoned back when they said they would, then there's the delay 6 weeks quoted and actually waited 16 weeks for half the job.
I suppose you're right, if I'm happy with the replacement pane, then I could pay for the windows. I just don't want that to get them out of completing the door in time, like I'm agreeing to changing contract. 
I've never made a huge purchase like this before. 

Aylesbury_Duck

Presumably you want the windows and the doors, so if you have the windows and pay for them, what's the worst that happens?  They don't supply the door and you don't pay for it.  Plenty of other places to buy a door.