New zip or not new zip?

Wow, thank you all, I wasn't expecting so many and varied replies.
There is an independent cobbler about half an hour's drive away, and I must admit I hadn't thought of him, even though I used his services in the past!
I must start with hairspray, that's such a random suggestion, it has to be worth a go!!
Sadly I am useless with needle and thread - the memory of needle craft lessons at school gives me the heebie jeebies - and I dread to think of the damage I could inflict on my boots and/or myself if I attempted anything in that way at my advanced age...and sadly I don't have a sister to sit beside me to spur me on.
I am willing to have a go at squeezing the existing zip. Will pliers be acceptable?
Ooh, so many things to try - thank you so much, you clever people

I read to squeeze the bottom of the zip with pincers. Then slid the zipper in. Idk if it works or not.

MYT62 guess it depends where you live we also have an independent chandler who always seems to have the items you can’t get elsewhere.

The trouble with putting a new zip in leather is that ideally you need to use the same holes.

My mother replaced a zip for me in some (tight!) leather trousers many years ago. I'm nearly certain she used a triangular leather needle.

She did it by hand.

I haven’t seen an independent cobbler for donkeys years.
What about an upholsterer?

Sago

Hairspray is your answer!

Spray the zip and allow to dry.

You may need to top up occasionally.

I promise it works.

Oh I have a bike jacket with a dodgy zip. I was going to get rid of it. Will give that a go. Just need a money off voucher for the hairspray 😂

Hairspray is your answer!

Spray the zip and allow to dry.

You may need to top up occasionally.

I promise it works.

A new zip on a boot would involve an awful lot of work and is not worth it for the cost per hours spent via a third party

Have a go yourself, its the sort of thing me and my sister would tackle. You would need a very strong zip and waxed thread, probably a strong curved needle and an awl to prod the holes. You work two threads at a time on leather, one from each side

Have you got an independent cobbler near by for a second opinion. The one in town will have a go at fixing anything and usually makes a good job at not inflated prices