@fury84 As a quick rule of thumb and to keep it simple and calculation free for you to put to your plumber 15mm has a capacity of 4kw, 22mm has a capacity of 11kw and 28mm has 20kw.
If you only have 8 radiators and lets say each one is 1kw each you have a need for 8kw to pass through the system to allow for the correct temperature drop for the return to the boiler.
As you can see 15mm can only carry half of your required peak amount so you would need 22mm to ensure the correct capacity can be carried while your new boiler runs efficiently.
Like I said this is a very basic explanation and a general rule of thumb but should give you some helpful info to put to the plumber.
Jon