Got an email from ANDY MACAULEY who says: there does not appear to be any gas leaving the cider bucket (sealed with a bubble glass filter). What have I done wrong and how can I start the fermenting process or is it too late? the apple juice has been ‘brewing’ for about 2 weeks.

Well Andy, for the first couple of days after pitching in the yeast there is no need for an air lock. the juice should be fizzing away madly and you should see and hear the bubbles. It should look like champagne, fizzing away.
If this was happening but then when you added the airlock it stopped, then maybe the air seal was not tight and the CO2 escaped through a different route.

If there was no fermentation at the start and there has been no CO2 then there has been no fermentation.

The cause might be as simple as a dead batch of yeast. I would normally say pitch a new batch in and see what happens. However, you say that the juice has been standing around for a couple of weeks now. I would therefore personally dump it and start again, just to be ultra safe. God knows what might have grown in there during two weeks.