How much extra can I run in a week off?

I normally run about 30+ miles per week at an average of 9mm-10mm, quicker for speedwork.

I have a week off coming up.  My boyfriend does not.  I would like to get out for some more long runs in this week, purely for the enjoyment.  If I stick to running very slowly does this decrease my chance of injury or is it too risky to run much more than 35 miles due to my current mileage?...

One week, no it will not greatly increase your risk of injury. If you are used to running 30 miles a week and have been doing so for some weeks or months then an increase of 15% is not excessive.

If you do run this week at an increased mileage then make sure that the following week is a recovery week, as you normally would plan for any race or event in which you take part. If you view the week as an event say and plan well then you should be fine and enjoy your time off.

I don't know, I'm not you.

I`m a teacher and I`ve made the mistake of overdoing the training in hols. Last July I ended up with a stress fracture.

Imho running very slowly as opposed to merely slowly or at long run pace will not really decrease your chance of injury. Slow build-up rules still apply. But not boosting the speedwork while increasing the other stuff temporarily you could probably get away with for a limited time. After all, a marathon race is typically a jump from 20 mile training capability to 26 and at a faster pace.

I don't have the data to make a probability judgment, but from a christmas runnng holiday I know that I got away with a temp hike from about 80 to 110 kpw. But I think there's probably a risk for that week, yes, which drops to normal when you resume your normal mileage.