You should be getting temperatures of around 85'f in the warm end and 75'f in the cool end. The temperatures you are getting at the moment are too low and could ultimately result in health problems.
The problem you are having may simply be that the type of heat source you have chosen isn't up to the job. I tried to heat my wooden vivs with heat mats when I first started out, and found exactly the same problem - I couldn't get enough heat into the viv! Heat mats have a number of downsides as a choice of heating. They only heat the substrate directly over the mat, they provide little or no heat to the ambient air, and more often than not the cool side of the viv ends up the same temperature as the room temperature the viv is in. Not good if you are trying to replicate a warmer environment for your snake.
Other issues are then thrown up when you try to use a heat mat with a wooden viv. If you put the heat mat outside the viv then you may get problems where the wooden base insulates the heat and stops enough of it from transferring into the viv itself. You could of course put the heat mat inside the viv, but then you risk the small but not unknown risks of possible electricution and thermal burns. It really is a minefield! It is important to say that some people do manage to create the correct temperature gradients using mats and wooden vivs, but from the posts on here just as many people struggle.
I would strongly suggest switching yuour heating method to ceramic. A ceramic bulb not only provides heat the the warm end, but also heats the ambient air and to a lesser degree the cool end also. This means that the temperature gradient is a true one, and not just "a warm patch on the floor at one end".
Complete ceramic bulb heater kit for reptiles, vivarium on eBay (end time 15-Jul-09 14:00:10 BST)
..is a great starter kit for ceramic heating. This will cost you just over £20 delivered, but if you do decide to switch to ceramic heating you must get the optional guard to prevent any burns to your snake.
If you haven't bought a thermostat (which are ESSENTIAL by the way) then you should consider a pulse-proportional thermostat to use with your ceramics. Any type of stat will work, but pulse-proportional stats are designed to work specifically with ceramic heating.
Triple 8 Reptiles - Microclimate 600W B2 Pulse Thermostat B2 PULSE
costs £30 and is a good stat from a reputable manufacturer.
Like I said, if you manage to get the correct temperatures using a mat then great, but having been in the same situation as you before, I can only tell you what worked best for me. Just to echo the above advice as well.. digital thermometers really are the best method for measuring temperatures. Make sure that the probe goes on the ground under/over the heat source.
Also, and forgive me of this is stating the obvious, but thermostats will never increase the temperature from a unstatted heat source, they will only limit the maximum temperature and maintain the set one.