Wholeheartedly agree Nick.
It also strikes me that we should take a look at all the other estuarys that lend themselves to this sort of thing.
My father (a Cornishman) tells the story of the Hayle tidal gates which used to be operated in order to flush the shipping channel clear.
It wouldn't be in the same order of magnitude as the Severn, but the key to this problem is "Every little bit helps"! (Not Tesco's)
I heard a caller on Radio 4 yesterday deride the initiative to replace all our filament light bulbs with energy saving ones as "A drop in the ocean" - but until the greater population wake up to doing the 'little things', they wont be in a position to decide coherently about the big things.

Off on a tangent for a moment, another caller on the Radio 4 program spoke about the scourge of mobile phone chargers and how people leave them switched on all the time (and only charge their phone for 2 hours every 3 days.

It seems to me the logical answer is to have a push-switch on the charger which 'clicks' out after the recommended charging period is up. A bit like those stairwell lights you find in communal areas. (You have 20 seconds to climb the stairs or you are plunged into darkness.)
That way, the phone owner can leave it plugged in & 'switched on' forever , it won't draw any current until someone hits the button - and then only for xx hours.

Then I got to thinking - how about making a that into a plug facility for all applicances. In the same way as you can get different rated fuses, you could buy a '1 hour' plug for your kids playstation, or a 2 hour plug for the TV.

Get me the patent office or Alan Sugar  - someone...