JBL still sell heating cables aimed for plants.
http://www.jbl.de/en/aquatics-freshwater-products/detail/4387/jbl-protemp-basis-250
And available in UK here.
http://www.zooplus.co.uk/shop/fish/technic_and_accessories/heating/jbl/13259
Interestingly it says gravel only as sand won't allow water circulation and is only 20W so really only enough power to heat substrate.
http://www.jbl.de/en/aquatics-freshwater-products/detail/4387/jbl-protemp-basis-250
And available in UK here.
http://www.zooplus.co.uk/shop/fish/technic_and_accessories/heating/jbl/13259
Interestingly it says gravel only as sand won't allow water circulation and is only 20W so really only enough power to heat substrate.