You are running 15W over what I would call a 30 gallon tank. The usual minimum for low light plants is 1 watt per gallon of water. Medium light plants go as low as 2W/gal. To be successful growing plants you will need to upgrade your lighting. The manufacturers of tanks sometimes will sell upgrade light strips for their own tanks so that would be worth checking into. They might be labeled as salt water hoods because salt water tanks demand higher light levels than fresh water. DIY light replacements are also made but I don't know about fitting properly into a UK hood since I have never done one of those. For an american tank, AHSupply makes some easy to use backfit kits that I have used. You can google that name and find them easily. They also have lots of information on lighting in general on their site so it makes good reading even if they don't make a kit you can use. I would expect someone similar to be doing business in the UK since planted tanks are popular there too.
