To not want to pay off my mortgage?

I think you don't save what you might never pay.

If you move house, surely, you'd need another mortgage unless downsizing. So you will very much pay 30k due to having less equity.
Up to you, really. I'd take paid off mortgage any day. I paid mine off 4 years ago (10 years ahead of schedule) and have since saved up which, together with my equity from a sale of my flat next year, is likely to buy me a bigger property either mortgage-free or with under 50k mortgage.

It sounds like your MiL is offering another £50k on top of £50k you already have. Why on earth would you turn that down?

How much is your mortgage a month? I'd set up a new account you can't easily have access to and funnel the would-be mortgage payment into that so it isn't frittered.

I was mortgage free at 30 as my parents gave me a 0% loan and it gives a massive amount of financial security and takes a lot of stress off if something happens like redundancy or illness. You and DH could always offer to pay MiL back so you don't feel like it's a random gift and still save on the mortgage interest.

We paid off our mortgage for emotional reasons. You are right it didn’t make financial sense but it felt good - I hate debt.

We haven’t frittered - increased pensions and savings.

Moving house doesn’t feel like a reason to me - are you worrying about being able to afford improvements on a new home? Surely you can just take out a new mortgage when you move . What am I missing?

Wouldn’t they? The sensible thing would be to pay off the mortgage and redirect the payments into your pension to benefit from the accompanying tax relief. Or do that with half of it and rebuild your savings pot with the other half. The psychological effect of owning your house outright can’t be overstated.

I'd pay off the mortgage, but that's mostly emotional I think. I dont know what would be better financially. Fully owning the house would make me feel more secure and I'd put what's saved on the monthly mortgage payments directly in savings anyway to build them back up.

If your mortgage repayment isn't a lot though then maybe that makes a difference to how good an idea it is?

I'd definitely pay off the mortgage. Then I would set up some kind of account where I transfer what the monthly mortgage payments were into a savings account.

To be able to save £30k in interest is massive

Because none of your other reasons really bear scrutiny at all. Obvs the thing to do is to save the mortgage you are not spending into a savings account (make it hard to access if you don't trust yourself)

You can't ask her to give you the money but not spend it on the mortgage, that is too cheeky

After FIL died my MIL gave us 10k as an inheritance that she wanted us to put towards the mortgage. Financially it didn't make the most sense, but it made her feel better to know that's where it went so we did it.

A financial advisor would not advise you to pay off your mortgage. But for many people it is an emotional thing.

You're going to have to explain the backstory a bit more if you want anyone to understand why you are turning down a £50k gift