Good luck finding any parts at RS even if the website's local store stock finder says so.
And unfortunate... I used to get almost all of my parts from them when I was studying electrical engineering.
They lost their way.. and it may be too late.
Their core market used to be the hobbyist/student/engineer.
As far as the lamp.... use the old grommet unless it's damaged beyond use.
A LED with a current limiting resistor works too but your going to have issue with the lens on the LED and getting the light more or less uniform.
If you go with another incandescent lamp make sure it's not too bright and that it does not get too hot.
I have seen lots of meters ruined because the lamp got too hot and discolored the meter.
LED would be a superior choice if you can get the lighting right.