Well i've been using this OMRON HBF306 monitor while at the gym...
I want to purchase the product so i can measure my percentage in the mornings, but to ship it over from the USA to UK will cost me an additional $60!!!
The best price i've found is: $38.95
www.homemedicaltestsmall.com (model HBF306BL)
The thing is that all devices used give you an approximate indication. What i recommend is that you use the same method at the same time of the day/week. That will give you a closer more correct reference to use.