It is unlikely that anyone really cares how much you charge them--what they care about it that everything goes well, so sit down and think good and hard about what it will take to put on two twenty minute Christmas sets that go well(one long set at a party is an invitation for disaster)--

If I may suggest--make sure your set list is up to the task, and that means Christmas songs,
not just Matty Groves and some random pop tunes, and remember that, especially for parties, your PA is the real instrument. Also, dress for success--you need to look like the 70 odd reveler should stop partying and listen to you--if you don't, they won't.

Since you work with these folks, you can joke with them and goad them in to singing along, and you can even make up a work related version of the 12 days of Christmas--"Five paid holidays" or some such.

If you make a special effort, you won't feel bad about charging even £375 or so,
and if it is a success, they won't care what it cost--