This US patent might be a place to start for a high efficiency combined solar PV and water heating panel which shouldn't be beyond the skilled DIYer.

http://www.patentgenius.com/patent/6630622.html

Packs of individual solar cells can be purchased quite easily (best to get the higher efficiency monocrystaline silicon cells)

All the other kit such as aluminium plate, glass, fresnel lenses, frame materials, bonding sealants, electrical, soldering and plumbing kit should be readily available at the local DIY store.