I'm with happy flyer.
The vast majority of boilers made by WB are top end boilers in their range - in my opinion they are usually in the top 2 the other being a Vaillant. I'd only say that on here though.
The after care in Northern Ireland is very good, the engineers are decent guys who will pull you out and depending when the break down is phoned through are usually fairly quick at dealing with the issues.
Here the out of warranty calls require a deposit of £250, but if you have multiple part failure then it is all covered for this price. Vokera do a similar thing - and if the customer can't get you and pays this deposit to be told he his pressure had dropped then there is no refund - before anyone says - he had been shown every year during the service how to top up the pressure!!!
I also like others try my best not to fit a WB, because I too think this accredited installers extra warranty is a joke. Worcester won't let me become one of these guys because "they had too many come on 4 years ago and they already are issuing too many 7 year warranties here". So for the same boiler if Dorothy is getting 3 quotes I will be offering a 5 year warranty and someone could be coming in with a 7 for similar money.
So as far as I'm concerned when i'm asked on the street, Vaillant are a better combi boiler, more reliable according to recent Which? Reports (a play on words this bit as they were more reliable until the most recent report Which? did which has the WB more reliable), have better after sales, can get parts easier for and didn't have a fan issue like WB did on their most common Junior Greenstar a few years ago.