The price of oil has got to the point where even LPG starts to look competitive, but yes, the payback time!
That's the problem. LPG siting costs and regulations can drive you mad.
Nearly 19 years is pretty good age for an oil boiler. Efficiency is well below that of modern units. My own choice would be to stay with oil, but upgrade to a new condensing oil boiler........ probably a Worcester Bosch.
Sometime in the next few years, I think you'd be very lucky not to have to spend out on repairing an ageing boiler.