UK Car Repair: how to change fule filter on corsa, plug holes, supply hose

Question
My son has put diesel in my 1.0 03 corsa.  We have drained the tank bought new plugs and petrol filter but cannot get filter off any ideas

Answer
Is the filter in the tank?  I would try starting the car. If you have drained everything out, remove the spark plugs, cover the plug holes with rags, remove the supply hose at the engine hold it over a container and have someone turn the engine over until petrol comes out of the line.  Whatever is in the engine system will blow out of the plug holes. Now put everything back and try starting it.