Finally got round to doing a quick dry compression test. Still need to do a wet compression test but the results are as follows:

1 - 8.5 bar
2 - 7.9 bar
3 - 8.0 bar
4 - 8.0 bar

When I took the spark plugs out, 1 and 2 had black residue, 3 and 4 were brand spanking new (as if I had just put them in and not started the car)

What would be the easiest and least messy way to empty the current fuel out of the tank? Also once the new fuel is in would it be wise to keep the engine running even though its misfiring to just use up all the fuel already in the system to see if the engine stops misfiring and starts running normally?