Took viscous fan off today in readiness to put electric fan in tomorrow but drove it back from garage today and the battery light on the dash is on permanent,any ideas

Hmm, that's an interesting one. Could anything on the alternator have got jiggled when you were working? Loose plugs, loose wires, that sort of thing? To tighten the belt, does your alternator slide in and out or have you got one of those spring loaded jockey wheels? If the former, it's easy to accidentally slacken things a fraction by accident when messing about doing other things under the bonnet. So maybe the belt isn't as tight as it could be.

New battery 1 year ago,new alternator 4 months ago and it has one of those spring loaded pulleys

You've either had a random failure or the plug at the back of the alternator is loose.

Pull and reset the connections, and then use a voltmeter and make sure you're getting 12 volts on everything there at the back with the engine stopped.

Alan