2004 Ford Streetka power loss when trying to accelerate

Check if this is the part you refer to ,

https://www.ebay.co.uk/p/1709854545

Hi thanks but it’s not the switch inside the car it’s under the car at the rear by fuel tank. I’m wondering if it’s an inertia fuel pump shut off

All fuel cut off switches I know of in various cars have all been inside and not underneath . Mostly done away with in newer cars and controlled by the ECU . In your OP you dont mention the plugs as having been changed which would be one of the first things I would do .

Hi all plugs changed and new Hd leads. Have also changed both lambda sensors. What do you think the rear part may be. It’s a small black box type with a small roundish part attached with a small pipe going into box and the other into round thing. The other end of pipe disappears under the plates etc going towards the front of the car.

Sorry cant recollect ever seeing this part on the few times years back when under a Ford Ka .

The small black box at the rear near the fuel tank is just a fuel vapour cannister. It's basically filled with impregnated charcoal and traps the fuel vapours from the tank. There's a purge valve on it so the vapours are recirculated and burnt via the intake manifold. By the way, the fuel pump cut off switch is located in the car behind the left side footwell trim.

Hi Sara
From my experience with power loss on the Ford Streetka, it was the fuel pump on it's way out. Another thing to look at is the fuel filter. It's a silver round cylinder with a pipe at each end mounted under the car near the fuel tank (on the left side of the car).
They should be replaced every 2 years or 24000 miles whichever comes first and is frequently overlooked when a service is done.
I'd change the fuel filter first (much cheaper than a replacement fuel pump) but if you do need to replace the pump I'd avoid a genuine Ford part as it's very expensive. A good quality replacement can be bought from somewhere like eurocarparts for a fraction of the cost. The pump is an integrated fuel pump and sender assembly and is fitted to the upper body of the fuel tank. To get to it, the fuel tank unfortunately has to be lowered or removed completely. Not as daunting as it first seems so long as you don't have a full tank of fuel as any fuel will need to be drained down first and a full tank holds 40 litres !
Good luck.