I have been in the habit of taking used potting compost to my
allotment and dumping it there. I can't do that now. My garden is
so small I cannot add any more to the level!
If I seive out the old roots, (eg of sweet peas grown in a large tub)
and add some slow release fertiliser or JI base mix, can I use it
again... for bulbs maybe?

Nutrients would be one concern, but you seem to have a handle on that.
My other concern would be a build up of pests and diseases.

To counter this I have tried:

microwaving - makes kitchen smell, and still got some 'extra'
seedlings appearing. Maybe I did not give it long enough.

soaking in boiling water - water cools quickly and you cannot be sure
that it is hot enough for long enough. Added to that you have the
problem of drying out your compost.

Next time I do this will try the wallpaper steamer. I think I will
need to invent a container to do my steaming in. One with the steam
inlet at the bottom. That way the steam should find its way through
the whole batch from the bottom up. My steamer does about 30 mins of
steaming per reservoir of water which should be plenty long enough to
kill any nasties.

Phil