What is this boiler, that it is 3 years old and has to be gravity (i.e. convection) circulated? Or do you mean just gravity fed, but with pumped circulation, in which case, any indirect cylinder of any type is fine.
I'm very surprised to hear that a boiler made in the last 3 years doesn't support pressurised operation. Do you have the make and model? If it can be pressurised, this is by far the most recommended route. It leads to quieter, more reliable operation and means you can remove your header tank, too. I suspect that you have a boiler that could be pressurised, but isn't, but I might be wrong.
Assuming you mean gravity fed and not circulated, then yes. You also have the alternative of a heat bank. The advantages and disadvantages of unvented cylinders versus heat banks has been done many times in the past and a quick Google Groups should come up with loads of stuff for you.
Christian.