Sorry it's taken so long for someone to get back to you, I'm far from an expert here but do the usual things like make sure the base unit is not stuck in the corner somewhere, and try to put in in the middle of the property that way hopefully you will get decent average coverage.
Also another thing to look for is a "cordless phone extender" google it and you should come with a few hits, also try http://www.pmctelecom.co.uk/categories/dect-phone-repeaters-%28range-extenders%29/36
No i don't work for them, also one last thing to consider but to be honest this is a last resort, look for some phones from outher countries as they use different freqs and higher power in some cases but having said that these are ILLEAGAL to use in the UK, if you live out in the sticks you may well get away with it but in a town or city you stand a higher chance of getting caught, but my best bet for you is the range extenders, they should do the job for you.
I live in a fairly large bungalow and am having "range" problems with a cordless phone. I dont particularly want to buy an outdoor phone as they are really heavy, but would greatly appreciate recommendations on a cordless phone with a fairly long range. Not interested in any fancy features.