I fear I've bought a bag'o'****e,need to replace leaking O/S Rear air shock due to leaking air,is this a D.I.Y job or am I to be milked!

You will get brand new bags on ebay for about £62.00 a corner - I would change both the rears if I was you, other one is likely to be on way out too and best to do in pairs
Dead easy job to change - will only take you half an hour or so to do both
TOP TIP - order some new locating clips as well, they are only pennies and are likely to be rusted in and may break