safe gas is only asking your location so we can recommend a forum member to you to price up upgrades or give a second opinion. In this case without doing the calcs it may be that your upstairs rads are adequate but are losing excessive heat to downstairs rooms. I would ask to see a break down of the guys heat loss calculations for each room to show you exactly what rads you should have.
On principle i wouldn't want to pay more as you paid and installed trust in his professional opinion. However, it is worth considering it would be a lengthy and difficult process to take the guy through the courts for larger rads.
I would advise getting a second recommended engineer to inspect your system, however, as if he has undersized the rads i would be consious over were else he has cut corners. Remeber an installation is only as good as the installer.
