Remove the carburetor float bowl and clean the float and needle valve and the main jet, the post through the center of th carburetor with spray carburetor cleaner. This would seem to be a carburetor that is gummed up. Here is a basic carburetor link.-----http://outdoorpowerinfo.com/repairs/briggs_walbro_lms_carb.asp
The small engine on this TESCO machine is described as a '98.5 OHV 4 stroke engine' and does not appear to be a Briggs and Stratton. There is an illustration of the lawnmower in 'Tesco Direct 'website and this might help you identify engine.
Sorry but I am not familiar with workings of engine. I know where carburetor is on the machine but I am unable to get at it. Don't have appropriate tools to dismantle. Probably best if I leave lawnmower with suitable service engineer. Thanks for your help.