Sounds like an infestation of some kind. I'm guessing aphids. You can use the sticky fly traps to find out exactly what is attacking your trees.

it could also be a form of scale insect, they are very good at looking like the bark. They also exude a sticky frass/honeydew. can you take a close up of the stems and post them with this question?

See if you can find a spray for this type of sap sucking pest. Bayer used to do one but don't know if they still do Bug Gun rings a bell.

Some scale insects can be killed by painting them with meths. A tedious job if there are lots but it does work.

It sounds like mealybugs, to me. While they are outside, natural enemies will keep the numbers down, but the greenhouse is a less friendly environment for the predators--too small--so the mealybug survivors' population explodes! Soapy water does help, but it may not help enough. Dabbing with meths does work, but, as Stera says, it can keep you busy for weeks. Commercial insectaries might have a predator suitable for release in your conservatory, but you will probably have to identify exactly which species of mealybug you have. Lots of work with a magnifying glass and internet pictures!

Thanks for all the help :)

Meths kills mealy bugs as well...cheaply and effectively. Done it many a time.