Thanks for all the advice.

Yesterday I managed to open the wound and release a lot more puss (yeuk) that and force feeding the antibiotics seems to have worked wonders.

She ate loads of tuna last night and even sat outside for a short while having a wash.

This morning she ran all over the house trying to avoid going in her carrier, so she's definately more like herself.
The vet gave her another injection and told me to keep bathing the wound until it's totally healed to prevent bone infection and to continue with the antibiotics.
Hopefully now she's eating I can put the antibiotics on her food to avoid any unecessary stress.
She came home and demolished a bowl of chicken so she's definately feeling a lot better.

Obviously nobody likes strangers entering their home (thanks for pointing it out Janee) and this is a problem we've encountered with numerous feline intruders over the years.
We have tried a magnetic cat flap but my cat didn't get on with it as it never opened so she was forever stuck outside. She also often charges through the cat flap at the speed of lightning so it wouldn't ever open in time. To prevent having a cat with a broken neck we changed it back to a standard cat flap.
We've also tried moving her food tray, thrown water over them and chased them out of the garden.
At the moment we lock the cat flap at night as we were often woken in the night by intruders coming up stairs and having a cat nap on the landing.

This isn't ideal as I don't want my cat to be a prisoner in her own home. If you can suggest something else I'd do anything to protect her.