Power Clamps, bar clamps, carpenters clamps or ratchet clamps from Roughneck are 6 inch or 150mm (ROU38256) 12 inch or 300mm (ROU38262) 18 inch or 450mm ( ROU38268) and 2 ft or 600mm (ROU38274) are a strong set of clamps. Buy them here.

6 inch clamp (150mm) http://www.diydoctor.org.uk/DIY-superstore/heavy-duty-ratchet-bar-clamp-6in.html
12 inch clamp (300mm) http://www.diydoctor.org.uk/DIY-superstore/heavy-duty-ratchet-bar-clamp-12in.html
18 inch clamp (450mm ) http://www.diydoctor.org.uk/DIY-superstore/heavy-duty-ratchet-bar-clamp-18in.html
24 inch clamp (600mm ) http://www.diydoctor.org.uk/DIY-superstore/heavy-duty-ratchet-bar-clamp-12in.html