why don't you get a quote from both of them and then decide? if the price difference is not a lot, go for the dealer option to keep your full BMW service history.
BMW will do you a good price as Z4 appears to fall under Value Servicing - http://www.bmw.co.uk/bmwuk/owner/service_value/overview/0,,1312___dht-aHR0cDovL3d3dy5iYXllcmlzY2hlYm13Lm5ld2RvbG4ubmV0,00.html
I got Insp II done on mine 3.0si @ 73k and it cost me 338.
Joe90, I have 3 mates down your way (guessing near Chigwell), all of whom have M3's and swear by a BMW indi out near Picketts Lock, they never use anyone else. I'd use him too, however I'm about 70 miles away.
I paid £317 for my inspection 2 from my local dealer . Got a few qoutes from different dealers then got them to match the lowest price.happy keeping the full dealer history for a few £ more! Indys were giving qoutes of £270.