Possible infestation causing itching, especially at night, please explain.

Are there any visible bites? If so what do they look like?

It could be mosquitoes, lice or bed bugs.
Another possibility is you have changed washing powder and are allergic to the sheets.

It's hard to tell without more detail. Also your location will make a difference to potential answers as some creatures are only foung in some areas.

Live in rural environment near Lincoln. Recently been on hols in Yorkshire, and also had influx of mosquitoes which I had been blaming. Getting 2 to 3 hard tiny lumps in areas, e.g. fingers or foot. Excessive itching at nights, husband affected to a degree. He considers bed bugs may be a possibility. Sprayed bedding with flea spray and found one 'apple pip' sized insect on sheet later on, too small for me to identify. No change in laundry products etc. Getting tired from lack of sleep.

Hmm are the bites in a line, this is common for bed bugs but not mozzies.
Their numbers increase in autumn in the UK. Look at the seams of your mattress and the back of the headboard this is where they commonaly hide.
Perhaps you managed to pick them up from where you went on holiday.

Another possibility - that I'll mention as you live in a rural environment is Martin Bugs. A lot like bed bugs but usually feed off house martins, they can attack humans if a nest is disturbed.

Good luck working out what it is and getting rid of it, hopefully some other people will be along soon with more advice.

The possible solution is due to entering of mite like the so call itch mite in to the host skin. And when it lay eggs the presence of those eggs can cause more itching.