Yes you're correct Hugh. Generally mountain bike insurance is quite rare I've found, either that or way over priced. The main problem is that high priced mountain bikes are targeted by thieves, and are easier to steal and on sell than an car for example - which tends to make the insurance companies rather wary of insuring them for a more affordable premium.
Most home contents insurance policies will pay out if your bike is stolen from your garage say, but usually they'll have a maximum payout well below the second hand value of even a mid priced well specced mountain bike. Typically their maximum default value is around the cost of an entry level mountain bike (intended for actual mountain biking use - eg Giant XTC and so on).
To insure your mountain bike for a higher value, you'll most likely be able to contact your insurance company and specify a more realistic value if you've got a much more expensive full suspension mountain bike. Bear in mind however, that almost all insurance companies, even once the value is specified as several thousand dollars, will only pay out an estimated secondhand valuation of your precious bike, even if its in virtually new condition. To get around this you'll probably need to find an insurance company in your region which offers dedicated mountain bike insurance - a rare find, and something you'll pay a lot for.
I've got several expensive mountain bikes which I'm needing to get insurance for... I suppose I will need to either get specific mountain bike insurance, or maybe just specify their values on my existing contents insurance... any suggestions?