Supernova,
Interesting question about diversifiers. During historic extreme stress on shares, property prices also seem to have been affected, and corporate bonds and most hedge funds were in the 2008 stress. Gold seems to have had a mixed record. Oil has kept value most times so far. Index linked bonds of short duration (or matching duration to how you would spend them) is propably part of the answer. Probably also important to consider diversifying within the low risk part of the portfolio.
BTW, I realised I said something potentially misleading above regarding FSA projection rates because they have already agreed to decrease them from next year to 3.5% real return for a portfolio 67% equity and 33% bonds. (PwC, who advised the FSA on the change, also came up with the same figure for a portfolio 57% equity, 10% property, 10% corporate bond, 23% government bond based on average real returns of 4.75%, 3.5%, 2.25% and 0.75% respectively.)
Another perspective on your current portfolio is half of the value is what you will spend over the next 18 years or so and the other half is what will grow into the portfolio you need to have in 18 year's time. Equities will always be a sensible choice for the latter.