1998 UK Ford Fiesta Mk4 won't start - sometimes

Car starts first time - no problems untill sometimes it just won't fire up.

When trying to start it on these rare ocassions of failure there is no smell of excess petrol, but there is nothing wrong with the fuel pump. There is a spark at the spark plugs. Checked out fuel line and replaced fuel filter. I've checked out earth wires to chassis etc. Then..., out of the blue it starts like brand new car..., until the next failure to start time...

Any suggestions for solutions

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