I guess £800 is bit to much for that kinda horse... if u check web u'll find a good horses for that price... not many but still... but if u see a potential in her... well... money means nothing if u really like the mare ;)

i also think £800 is a fair price for this horse.  i wouldnt go much higher tho but its up to you.

This horse sounds like she needs some TLC and a dedicated owner to help her adjust.  It depends how much you really want her, any item including a horse is only worth whatever a buyer is willing to pay.  Head shy and nervousness can be overcome with patience and kindness, windsucking/crib biting not that big a deal, many people say it is but it's not, I once had an ex-racer who windsucked/cribbed and occasionally weaved and it was only an issue for other people, other horses absolutely do NOT copy it despite what some people will tell you, I can guarantee it.  The scar's only a problem in showing where they have to be beauty queens, it won't matter in jumping, dressage or cross country.  5 years old is also a good age to break her, she's going to be quite mature physically and ready to work.  You could end up with a good horse, if it were me (and I'm no business woman) and I really did want her then I would maybe go up to £900 but stress that it is absolutely the final offer.  You'll know yourself how much you want her.