Sadly not the problem in my case....
just tried a similar trick by putting insulation tape over pins 1,2,5&6 of the RJ11 plug, but same result - a pulse when I plug in and that's it.
Just to do things properly I dug out an old RJ11 cable and the multimeter and checked the state of the socket on the gas meter - of the 6 spaces for pins in the RJ11 socket there are 4 connections in positions 2,3,4 and 5. As far as I can tell, 2 and 5 are always connected to each other, as are 3 and 4. This ties in with the probability that my reed switch has bitten the dust...

On the RJ11 plug from the GaSmart, the pins are as follows (sit the plug with the pins facing up, wire going away from you, count pin 1 on the left, 6 on the right
1: Not connected
2: +3V (yellow wire visible inside the plug going to this pin)
3: +3V (green wire)
4: 0V (red wire)
5: 0V (black wire)
6: Not connected
Connecting either of the 3V pins (2 and 3) to either of the 0V pins (4 and 5) seems to give a pulse for me. This would explain automan's issue - if his meter internally has 2 connected to 5 then you'll get just one pulse on plug in, even if it is happily switching 3/4 as the gas is used.
On the plus side, I have a cunning plan...

The only thing in the house that uses gas is the boiler - it has a burner light on the front. I've designed a circuit that should detect that light being on and will connect to the RJ11 plug and send pulses at a constant (but adjustable) rate while the light is on. It won't be perfect as the boiler can reduce its power under some conditions, but it's better than nothing. When the bits arrive and I've got it working, I'll post the circuit diagram here in case anyone is interested.
cheers once again!
Robin