I politely turned down the quote to sort the seized dustcap 🥴

While that is a ridiculous quote, this will probably cost more than you think even at a tyre garage.

Not much chance of getting a metal dust cap off even with penetration spray.  Realistically looking at a new valve, which means tyre removal and refit.  I wouldn't be surprised if it was £30+.  And that's assuming they don't damage the TPMS sensor in the process.

I had 4 metal dust caps seize solid on my Focus tyre valves. While they looked nice with the Ford oval on them, I'd never use them again.

Took me over an hour to get all 4 off using a junior hacksaw blade and carefully cutting down the length of the cap until I just about reached the brass threads of the valve. I was then able to 'split' the valve cap with the use of some wire cutters. A slow painful job. I now have some standard plastic caps 👍

Ta for the replies, was thinking of using a Dremel cutter (carefully) on the length of the cap then split it like you did 🤞