Owning your own home

It’s a private block. The management co are c**ts (but aren’t they all). The work is genuinely needed tbf but the money they want seems way too much to me.

Well, there are clueless cunts and there are overtly hostile freeholders as @BleakRefs will attest to. If you have a share of freehold you have a bit more power to scrutinise what is going on. For example, you could talk to residents on the board of directors. As a leaseholder you could pay attention to the S20 process, But it sounds like you either missed that or are resigned because...

...I am happy to pay it but would prefer to pay it in instalments. I am wondering really if they have to accept that offer?

I suspect no - they do not. For various reasons. For example, they might need all the money up front from all the leaseholders to even proceed with the works. You could try https://www.lease-advice.org for more bullet proof insight?

Thanks for the tip about my mortgage lender, didn’t think of that. Do you mean I can just add it to the mortgage?

Depends on your mortgage. If you've been over paying for example, you might be able to extract that money back. Or they might let you take a payment holiday. Or if your fixed term is coming to end you can re-mortgage and take the money out again. It depends.

Ultimately, if you are a leaseholder, and there is no sinking fund, you must have some way of quickly raising the money to pay for Major Works. A personal rainy day fund, flexible mortgage, bank loan etc.