upstairs radiators cold downstairs hot?

i have a glow worm compact central heating system just recently the radiators on the first floor of my home do not get warm whilst downstairs there is no problem. I have been told by a plumber that the pipework 10mm feeding radiators is too small and should be 15mm. I have lived in this house since new 8 years and this problam has not happenned befor. Is the plumber conning me to justify the exhorbitant cost of unneccesary removal and replacement of supply pipes