Your questions seem to have been answered but I will also chime in here with my minimal experience. I've only had my girls for a couple of weeks so have hardly any experience, but it's a steep learning curve and I've learned a lot already!
1) How tall does the attached run need to be if its enclosed?I strongly agree that it needs to be high enough for you to stand up in. Makes it so much easier to clean- and of course if it's easier, you're more likely to do it even when the weather is telling you to stay in bed instead.
2) If they free range during part of the day, is the lawn likely to be destroyed (more so than by the children and rabbits!)?They do like to eat grass and greenery, and to scratch in the lawn. Whether this 'destroys' it is a matter of opinion. My lawn wasn't great to start with so I think long term having the chickens aerating and fertilising it will actually do it some good. If you have a vegetable garden, consider fencing it off with some bamboo stakes and bird netting, as they will definitely get in there and destroy it!
3) Is free-ranging hen poo a problem?!Again, depends what you consider a problem. They do poo
a lot and their poo can be quite stinky (although this may depend on what they're eating). I used to wander around the lawn in bare feet but don't do that any more! And all shoes now have to be taken off at the back door before anyone steps on the carpet! I'm considering dividing the lawn up into areas that I can fence off in rotation so the mess is more confined (I have 6 pullets who are currently free-ranging all day).
4) Will they escape over a low fence? (Side fences to the neighbours are high but at the end of the garden there is a low fence separating us from the fields behind our garden)Depends what you consider 'high' and 'low'. My chickens haven't gone over the 6-ft boundary fence (yet!) but would easily get over a 4-ft fence and I'm sure they could do the 6-ft one if they had an incentive. I've had more trouble with them squeezing out through small gaps, and using other objects as steps to get on top of the shed.
5) How long is a chicken's lifespan?Others can answer this better than me. I chose heritage breed chooks with the intention that they may live for 10 years. A woman I know had a bantam who lived for 15 years.
6)Could a static coup/run be kept on a hard surface if the chooks got time out on the lawn at some point in the day or should they always be kept on earth/grass? A hard surface covered in bedding will be fine. My local council regulations specify that the coop must be kept on a washable surface (eg concrete) so technically I'm in breach. Don't tell them!