Welcome, great first post
Plasma or LCD?
LCD is by far the more common and the latest models are pretty impressive especially in the size range you are looking at.
Is it true that Plasma is

in a well-lit room?
Nope, however glare can be an issue on some plasmas but no flat panel is perfect they all have their issues and even CRT if the sun is shining just right can be hard to watch.
Which makes are best value?
Tricky, LG and Samsung do decent stuff at decent prices but value is hard to define.
Is Panasonic worth the money?
Yes, I think they offer some excellent products at competitive prices.
Is buying online safe (and what do I do if there's a problem?)
Yes if you don't do anything stupid

Reading forums for other peoples experiences with online dealers is a gold mine in making the right choice. That said a TV is for life not just for christmas (well you know what I mean) so if possible you should try to see any TV in the flesh.
Paying by Credit Card gives you added protection, manufacturer warranties are more reliable the retailer backed ones but that is the weak spot in online purchasing, no immediate one on one interplay.
Do I need to pay a fortune for leads?
Nope.
What the

is blu-ray (apart from being a mate of Superman ).
It's an optical disk the same size as a DVD but thanks to using a Blue laser instead of a Red laser (like DVD) the disk can hold a lot more data which means they are used to store HD programming.
You need a Blu-ray player to play the Blu-ray disks but those players also play CD and DVD disks.
Which hard drives are value for money - or should I get the more expensive (and more complicated) alternative, the name of which I can't even remember!?
I assume you mean a PVR which has a hard drive built into the box so you can record either Freeview or Freesat internally using two tuners?
Freeview PVR's are good value, Freesat being more recent hasn't the mass market presence to force the price down and SKY and VM PVR's are subsidised or loaned out.