Hi Chrichtons and welcome to GoY I'd hack it back hard after flowering. This clematis flowers on the previous year's growth and I wouldn't expect to see much new growth yet but if it is getting out of hand cut back quite hard once flowering is past. I know in theory this is the only time you should cur back a C montana but we have two thugs that need cutting back several times and still flower like made the next year, we remove up to a third of the new growth in July and September otherwise by now they would be in next doors garden!