pousser, pruning now would give your plant plenty of time to produce new growth and for that growth to mature before winter. I couldn't say if in your climate you would still have bud set for flowers next year, but losing the flowers would be your only concern with pruning first week of June.

Thanks for reply morz8 - I'm still trying to pluck up courage to do this. I'm thinking of tackling selected branches, maybe over 25% of the plant, particularly those that are pushing ahead of others. I guess, then, those uncut branches will at least leave me with some flowering.

I think you would be OK if you prune before the end of June if the plant gets 3 or 4 hours of sun on a sunny day. The bud set might be a little less, but should be adequate.