Nissan Qashqai Problems

[Lisa30]

Hello

I have a 2015 Nissan Qashqai 1.5 diesel, about a month ago my engine management light came out and I had a garage diagnose the fault and the code was P0380 which indicates something is wrong with the Glow Plug / Heater.
The garage cannot confirm the actual fault, but they kept my car overnight and wanted to start the car in the morning whilst the car was cold and the car started up straight away, but what else could the problem be?

Whilst I am driving and in acceleration when i am in 60mph I feel the car power reducing slightly, even when my foot is right on the floor after a few minutes the power comes back.
I have also noticed my diesel is being used really quickly than an economical car should be..

Any help or guidance would be greatful.
The garage say they cannot look at my car this week but asked me to bring it in.... so i am looking to see if anyone can help with the problem so i can ask another garage.

Thanks in advance.

[feef]

I'd expect a glow-plug fault would only manifest on cold start. After that the plugs aren't used so shouldn't affect normal driving performance.