Oh dear, you have my sympathy. You need to call and wo/man who can. Do you have breakdown cover? They usually offer a fuel draining service. i know the RAC do for example.
How much ? A small amount is not a problem . In fact people used to put petrol in the diesel tank during cold weather to stop the fuel 'waxing' and blocking the fuel filter .Other wise there should be a drain tap on the bottom of the tank to empty it,use that .
Anything else is likely to be dangerous and illegal (because pumping out, storing and disposing of highly combustible fuel needs to be done in accordance with strict safety rules).