Limp Home question

My 2.0 CDTI 2015 85K miles Astra was perfect yesterday.
This morning it just had no oomph. No acceleration.Top speed was 30.
No warning lights.

I took it for a long drive along A roads with 70mph limits and it eventually got up to 60, but with no real acceleration, and once I turned off it was back to sluggish mode with people flashing me from behind to go faster

I drove home, pushed in the connectors around the top of the engine and left it for a couple of hours while I phoned a local garage who said they would check the diagnostics.
I left home to take it there and the car was back to normal.
They put a tester on it to check past errors and said it was the fuel rail pressure sensor and would price it and let me know.
Was this the cause or the symptom?
Is it an expensive job?

It also has had the SVS warning up for ages and no Start/Stop. They told me it was the pressure type gloplug on cylinder three. It has been running fine but i will get the two jobs done together if necessary.
The garage cannot source this type of gloplug, any suggestions where I can get one at a reasonable price?
Thank you.