Hi
I changed my boiler over a year ago and it cost £2600 for a vaillant combi. But that included removing the old system, redoing some of the pipework, powerflush, a magnetic cleaning device and pump to drain water. The problem with replacing an old system is that the old gas pipe size is not sufficient to supply most new systems. My boiler had to be moved to another part of the house.
It depends on what you can afford. Some of the reviews online for the Vokera are a bit on the negative side. The £1600 quoted appears very cheap. What work does this include exactly? Can the new Boiler be fitted in the current position?
The makes which most recommend are, Baxi Potterton, Worcester Bosch and Vaillant. If you speak to Independent Gas Boiler installers and service engineers most appear to recommend Vaillant, but they can be more expensive. The benefit of having the Vaillant system solution to me, was that I had my boiler placed in the old airing cupboard. The vaillant can be supplied with a pump to remove the excess condensed water and the water is pumped to an internal drain point. With other systems an ugly draining pipe has to be added to the outside of the property. A neighbour had a cheap system added and there is a horrible pipe going around the house.
My advice is to look into it a bit more and then buy the best that you can afford. Make sure you are happy with the way the new boiler is going to be installed. Insist on a powerflush of the system before new boiler is added.
Hope this helps.
Huckster