Graemeb,
70k is just about enough to live on, but you will not be able to save anything. So make sure you have a return ticket or some funds tucked away for a rainy day.
A flat in Colombo would star from about 25k.
Internet landline 1500-2500 rupees a month, if fixed landline. You can olso get a dongle and pay as you go. Without heavy downloads around a thousand a month should be plenty.
Local food is cheap (bread 60 rupees), imported is more expensive, just a tad cheaper than the UK.
Visas... you need a company which will provide the residence visa support. It is for one year, and with the proviso that you only work for that company (it's be mentioned in your passport visa). Tourist visas are 24 hours online, and can be extended every two months for up to six months, after which you'd have to leave and re-enter.
Hope this helps.