nope it doesnt cut out?

You can buy a little neon tube spark tester which goes between the
plug top and its lead. It flashes (best viewed in shade) when there
is a spark. Cost about 3 quid or less.

In open, good ventilation, attach clear plastic pipe to outlet of fuel
pump, lead to jar or bottle. Crank engine, fuel should be delivered.
Piece of pipe about a quid.

Now when it won't start, do both checks.

If you have fuel, and ignition spark, engine should start, or
at least try to start.
It could be the ECU, but they don't usually repair themselves,
so the garage can't find anything. Breakers yards are a good
source of ECU's if you know what you are looking for.
Other problem sources are
fuel cut-off sensor (bad contacts or faulty) fitted in many
modern cars to cut fuel in event of a crash.
lambda or MAP sensors, though again these don't
tend to repair themselves, but can suffer from bad connections.
Crank sensors and their connections can become intermittent,
they drive the ECU to generate the spark and time the engine.

Problem is it all takes time, garages charge by the hour,
if it is intermittent it never shows up at the garage, and
eventually (at garage labour rates) you go bankrupt.
Does you local night school do engine maintenance
classes.?

There is a rubber pipe link between tank pipe and engine
fuel pipe to take up flexing. If perished it gets porous, air
can leak in. Fuel drains back to tank.
Crank engine. At cranking speeds pump doesn't pump
air very well, particularly if air is still leaking in.
So you crank and crank, it doesn't pull enough fuel
from the tank, battery goes flat, cranking gets slower,
it won't start. Once you get it started, at engine speed,
fuel gets pumped engine runs fine.
Two, or so, hours later, or overnight, won't start.
The pipe is usually somewhere near the fuel filter,
back of the engine, between engine and bulkhead,
or clipped to bulkhead
It's a long shot but worth checking.

Find a knowledgable boyfriend, feed him well.