As per the title, the damn cassette will not come off. Any suggestions please?

you’ve got the lock ring off?

Is is corroded on or is it an alloy freehub and the cogs have cut into it?

If it’s the latter and the cassette is being replaced then I’get the freehub off the wheel and then cut the cassette off.

I found you can pry off the single sprockets, which are usually bitten in and then clean the burrs and slide off the alloy web part. Maybe easier if you can to remove the cassete and freehub and bung it in a vice.

If the cassette has dug into the freehub then I find the following releases them:

Chainwhip on larger cogs and tap the cogs with a screwdriver/hammer combo. Do the same with chainwhip on small individual cog and tap round the main cassette body.

Hope that makes sense.

If its sprockets gouging an alloy body I use a 2nd chain whip to turn the individual sprockets anticlockwise to free them up.

Hit the cassette with a hammer, it may not work but it always makes me feel better. 🙂