hi, i need to replace the expansion tank on my mk2 focus, as i have tried to get the old part of the cap out but no joy, my biggest issue to making sure i dont get trapped air after i fit the new one, is there a way of making sure this doesn't happen, thanks Steve

There's no completely failsafe way to do it at home.  (The proper way is vacuum bleeding using expensive equipment).

Best chance at avoiding an airlock for this job is just to not drain any more than absolutely necessary.  Keep the pipes as high as possible at all times, if they drop just once you can quickly lose a load coolant which is replaced by air.

Once refitted, fill the new tank to max and run the engine with the cap off and the heaters on (but blower speed low) until the thermostat opens, and ideally until the radiator fan kicks in.  Make sure the heaters are blowing hot air to confirm no air in the matrix pipes.  If the coolant level drops in the tank, keep topping it up, making sure it never drops lower than the min level, as that's when air can get sucked in which ruins all of your earlier work to keep it out!