Hi,
I too am in the UK and there are a lot of dealers out there that will welcome your business so stay away from your original dealer.
I have a few machines and one of them is the Janome 4900 which is the next version to yours. What I would suggest is talking to Janome or taking your machine to a different repair/Service place. Whilst I agree that Janome's can be a tad temperamental with regards to threads, once threaded correctly they work like a dream. Always ensure that the two green lights on the front of the machine are lit before threading the machine as this indicates that the machine is in the optimum position for threading. Try asking other people where they get their machines serviced..... Word of mouth is always the best form of recommendation.
I hope this helps..... If you want to chat some more about this, please feel free to PM me.