GH writes
There was no warantee; but I'm only hoping to get rid of the mites now.
Do you know if the advice to spray with a fish meal or kelp solution is
of any value?

If I change the earth, will I be able to use Phytoseiulus (if nothing
else works)?

No, the systemic insecticide you have used will be in the plant rather
than the soil. You'll be able to use biological controls eventually, not
sure how long you need to wait.

I'm not sure how well the biological controls work on the domestic scale
- I suspect they will give something like a 99% clearance, which is OK
when you're using them routinely every year in a greenhouse, but the 1%
(or whatever) that they miss is enough to start up a new infestation.
--
Kay