If the gas supply is switched off from the mains, but the boiler control unit program is still set to run. Will this damage the boiler??? Will it just sit there sparking away?

It will have a safety cut-off if it is a modern boiler.

It's 17years old

Turn the boiler off too? There should be a electrical fuse or safety switch and then it wopn't even attempt to self start.

I would switch it off, but unfortunately I am already away whilst the gas is being turned off i.e. I forgot to switch the boiler off.

I think in old boilers that can cause overheating of the pressure chamber – probably explode mate. 😯

Oh well soon see 😆