Here is a link to a thread I posted in AIBU a few months ago. I am the same OP but I've got a new phone and can't seem to log back in, despite reset emails! not very technical : www.mumsnet.com/Talk/am_i_being_unreasonable/2569277-Builder-sending-a-new-invoice-after-work-is-finished-and-balance-is-paid
Basically we had major building work lasting about 8 months. We paid every invoice immediately throughout, including the final one back in December.
In January, the builder sent an additional invoice for nearly £1600 
We were having our 2nd baby less than a week later (elcs) and called him to say we disputed it and had other priorities, but would respond.
A few weeks later he sent a chaser email, asking for the balance asap. We responded within a fortnight, apologising for the delay - a beloved family member had died suddenly and what with that, toddler and newborn it had taken us a couple of weeks. We sent a detailed response to each item on the invoice and offered a full and final settlement of £378, largely as a goodwill gesture.
Nearly 4 weeks later he replied saying that he didn't accept this and we should pay in full asap.
We replied within a week with another detailed response and offered slightly more, but nowhere near what he was asking.
Some of the items are extras that he mentioned at the time and then never got around to pricing and invoicing us for, despite us asking. We've offered to pay for these things.
Other things though are things like rerouting drainage in the kitchen. He says that he didn't know the intended layout, and so based his quotation on our previous kitchen layout. This isn't true. We showed him detailed scaled drawings of the new kitchen before he ever quoted, and the new kitchen was on site for him to see (in the garage).
His plumber didn't come to look at the job before he quoted, so he guessed on his behalf. I think that the incredibly slow to do anything, delayed the completion by months plumber has now invoiced the builder and he's trying to pass the cost onto us.
It's been weeks since we sent him our final offer. I'm really upset and worried and don't know what to do. I dread checking my email. It's a lot of money and we need to know what's happening, especially as I'm on maternity leave now.
I feel like this whole business is an intrusion on our family time and I just want it done. I'm worried it's going to get nasty and confrontational. I don't think he would but I dread him turning up when dh is out. He's best friends with our ndn, which is how we found him.
I'd be very grateful for any advice please. How long do we have to reasonably wait for him to respond? Can we say that we revoke our offer if he doesn't respond by X date? We sent everything without prejudice.
Thanks!