Thanks for that reply John, most useful!
I have options on two second hand 3530LS tripods. One is priced at £250 (the private seller said there is only a couple of marks to one of the twist locks), and the other priced at £350 stated as mint from a main Gitzo dealer in the UK.
I purchased a mint second hand Wimberley II head from MPB Photographic earlier this week. There is a card in the box dated March 2013 so I suspect the head is quite recent. However, replacing the 'Torx' bolt is a good idea - I've read elsewhere the original has a tendency to become loose over time if not checked regularly (which would be good practice to check the tightness in any case given the payload!)