I know its all changed now but do they still allow centre holds between the runways, or is Heathrow totally restricted.?
Yes they might still ask you to hold between the runways if they have cleared you to cross the departure runway but have approach traffic on final, usually it's not required the other way around as you'll be cleared to continue downwind of the departure runway threshold. So if required you are instructed to hold between the midpoint line between the runways and the departure runway threshold i.e no closer to the landing runway than the midpoint between runways. Once clear you'll be cleared to cross the landing runway as expeditiously as possible.
http://www.ead.eurocontrol.int/eadba...2011-12-15.pdf
addition - you are unlikely to get a crossing if TEAM operations are underway during busy periods (landings being made to both runways simultaneously)