Can anyone advise on what Plexr actually is, what type of treatment?

Plexr is a device that produces "plasma" just above the skin surface. Plasma is essentially excited, ie high energy, gas molecules which deliver heat to the skin surface. With the Plexr device it will produce a small dot on the skin. By delivering a series of these over an area (the eyelids are a good example) the end result is a degree of skin tightening along with firmer smoother skin. 
It is important to have a consultation to establish whether you are likely to get a good result, but in my experience by selecting good candidates for treatment the satisfaction level is very high.
Side-effects - there is crusting over the area which takes about a week to settle, and there will be swelling for the first few days.
Risks - when performed properly it is very safe, but in theory it would be possible to cause a burn or scar; it is important to follow post-treatment instructions as there is a risk of developing pigmentation (usually temporary) with sun exposure following treatment, especially in darker skin types.
If you are thinking of having treatment around the eyes it would be worth looking at Tixel as well - this is more comfortable and causes minimal side-effects, but does require more treatment sessions.
Good luck,