Should I charge my PHEV to 80% to preserve the battery?

Can you advise on this classic 'down the pub' argument please? Is it best to only charge my plug-in PHEV BMW X1 to 80% battery capacity and forgo the lost 'electric miles' in order to 'conserve' its battery...

Do plug in hybrid cars need the same charger as a full EV?

Do plug in hybrids need the same charger as a full EV or can a smaller capacity charger be used?

Can I charge my PHEV from a household three-pin socket?

I have purchased a plug in hybrid which give 75 miles on electric. Can I charge the car from a domestic 3 pin plug?

What happens if I don't charge a plug-in hybrid?

I'm thinking of a 1.4 plug-in hybrid Skoda Superb. What happens if I don't keep the battery topped up?