If there’s water in the tank, you should be able to see it when you look inside (may need to shine a torch into the oil). It gathers in pools at the bottom of the tank. The outlets on plastic tanks are usually raised from the bottom of the tank, so it would take quite a lot of water before it started coming through the tank outlet.
Do you have an in-line filter fitted? If so, water should accumulate in that first. Ones with clear bowls make it easy to check for water, otherwise you can (usually) open the drain screw and see how much, if any water comes out. In line filters are <£10 if you haven’t already got one, or fit a dedicated water trap for a few £s more (as @paton posted above).
If there isn’t any water in the filter, then it probably isn’t what’s causing problems with the boiler…
If you have a small amount of water in the tank, there are water dispersant additives that you can use to get rid of it – e.g:
https://www.comstarproducts.com/water-absorber-tm-water-dispersant-for-fuel-oil-1-pint-68
(I haven’t used them on heating oil, but have used something similar to clean up a diesel tank in a boat).
If there is a significant amount of water in the tank, you might be able to remove much of it with a vacuum extractor and a long pipe to reach through the oil to the water (and then treat the remainder with a dispersant), or, I believe there are water absorbent ‘pigs’ that can be used.
We’ve had a problem with lock outs on our boiler.
We had random lock-outs on our oil boiler (a Riello burner). It always fired up again first time when reset. Water or dirt in the oil was suspected, but when the pipe supplying the burner was disconnected (gravity feed) the oil flowed freely and what was drawn off was clean and water free. It turned out to be the motor run capacitor that had degraded, resulting in the burner failing to start every once in a while. What should have been a 4.5uF capacitor measured at 2.7uF. Replacing the capacitor (about £20) cured the problem completely. It turns out that this is quite a common problem, so I would hope that your engineer has considered this.