In message .com, Ed
writes
I am told the best time to do this is just after its flowers have
fallen (a couple of weeks time?)

Is this correct, and how severe can I be? I'd really like to get it
back to a bush sized about 6 foot tall, but it has grown about 10-12
foot tall and takes up a whole corner of the garden

cheers for advice
Ed

The usual advice is to cut out a proportion of the stems at their bases
each year. So, if you cut out a third each year, in three years time
you'd have a bush about 6 foot tall.
--
Stewart Robert Hinsley