Hi Ian, I recently re-sealed my windows both inside and out and its not really that hard to do.
I popped down to what's known as the Fethiye B&Q and got myself sealant and pump gun. Some of the windows were worse than other so anything that was too loose I completely removed the old silicone, cleaned it and vacumed out the debrit, and then pumped in the new silicone. I smoothed in the new silicone by running my finger from the top to the bottom of the window and then cleared the exess with a damp cloth.
I tend to do this job every year and with the windows that are not too bad, I just add the sealant where I think its needed rather than clearing the lot (like above). If you are not confident about doing it, try it first with a small window to see how you get on.
Hope that helps.
Steve