[chris (OP)]

What is the jetstream, what causes it, how high is the jetstream and how does it affect the weather in northern Europe?

[evan_au]

There are several jetstreams in both northern & southern hemispheres of Earth (and Jupiter).
The one most affecting Europe is the Polar jetstream, which is 7-12 km high.
It occurs on the boundary between large-scale weather patterns which transport warm air from the equator towards the poles. The boundary between the Ferrel cell and the Polar cell is where the Polar jetstream forms. But it is a rather meandering pattern.

I started here: https://en.wikipedia.org/wiki/Jet_stream

The weather in Australia is more affected by El Niño/La Niña  years - and I see that it's widespread effect also impacts the path of the jetstream in the northern hemisphere.

[Colin2B]

As @evan_au  says the jetstream forms at the boundary between the warmer Ferrel cell and the colder Polar cell. In the Northern Hemisphere this boundary is around the same latitude as the UK. The boundary is where depressions form with their characteristic warm and cold sectors, and warm, cold and occluded fronts which dominate much of UK weather. You can usually tell where the jetstream is because, as the depression forms,  it is above the junction of the cold and warm fronts at the northern end of the warm sector.
If the jetstream is running to the north of the uk, which it tends to do during the warmer months, then the depressions will track over the north of the UK leaving the south with  fair weather. If it tracks south then they pass over southern England bringing typical depression weather.
Sometimes an area of high pressure will move N from France/Spain and act as a blocking high preventing the depression moving E and bringing prolonged fine weather.

[jeffreyH]

Colin2b is a weather man, a weather man, a weather man. Colin2b is a weather man an so is evan_au ...
https://en.m.wikipedia.org/wiki/A_Tribe_of_Toffs

[Colin2B]

or you could try Crowded House "Weather with you" or perhaps "Four seasons in one day", which is what it feels like sometimes.