If the problem cause the boiler to goto lockout only after long off periods it is most likely down to either an air leak in the fuel supply or the fuel pump is on it's way out.
The air leak inthe fuel supply can easily be checked with either a vacuum pump or pressure pump.
An experienced oil engineer should be able to sort this out quite quickly.
It is not uncommon for oil pumps to fail after a couple of years especially if it has to cope with an air leak on the fuel supply.
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