B&Q lawnmower stops after few minutes

i have B&Q brand lawnmower, one of the cheap ones with plastic body, it's about 4 years old. It has reveed up and down for about the 2 years but working ok, I'll admit I haven't been looking after it as well as I should have. But I brought it out for the first cut of the year and after about 20 minutes it stopped. It started again but died again after 5 minutes. Each time I started it, it would last a shorter time.

I checked the air filter and fine, I changed the oil, I took out the exhaust to make sure it was not blocked, I took apart the carburetor(I think) and made sure it was clean.

I tried again and ran for a bit longer again but eventually died. I ran without the fuel cap and without the air filter and still died.
It sounds like it's running out of petrol but I've checked as much as I could and looks like all is clear.
Any ideas?
If not I'll take to repair shop, will they even touch a B&Q mower and if so what do they charge?