I have done this without removing the headlamps. I used a smallish drill and easily accessed the boss with the bonnet open.
I used a small pump (fish tank size) with an 8mm pvc tube and the suction port to vacuum out the swarf
Then I super glued the "T" pieces for added assurance.
This solved my problem completely.
I did have a blemish on one lamp due to past condensation leaving a dried patch, but this is of no concern.
I posted this in 2008 on another Jag site.
The whole job cost less than £2 and took 20 minutes only.

Good luck, let me know how you get on

I have the workshop manual and it took me a while to work out that to remove the headlights you need to remove the panel from inside your wheel arch! Once I worked that it it was easy to remove.

Don't want to make you too nervous, but check out the price of XK8 headlamp units before starting work.
Cheers