if your boiler ain't broke then carry on using it until it's no longer cost effective to repair, by getting a new boiler is going to save you between £100-£300 a yr in gas which may sound alot but if you consider the cost of a new boiler this may take you 10 yrs to recover, the avg life of a modern boiler is 10-15 yrs so by the time you have recovered your costs it may be time for a replacement anyway.
the only thing going for megaflow is they have an internal expansion system, i fit ariston cylinders, they do req a seperate expansion vessel but they still come with a 25 yr warentee (just may sure you get the stainless steel one).
have they tested your incoming mains for flow & pressure ?
both of these things are mega important because if either are poor then an unvented cylinder may not be the best option for you