Just my :2c: but changing the fuel filter i doubt will make a blind bit of difference. As the problem is intermittent and the fuel filter would be classed as a mechanical item (not electrical) i would say it highly unlikely to cause a misfire. The pumps are able to run at pressures exceeding 7 bar, which is more than adequate, as the standard running pressure i think is only 3 bar.
Changing the fuel filter on a metro would be different. Yes, it is exactly the same filter, but, the metro fuel tank is located underneath the car, and also made out of metal, which will obviously corrode and cause crap to form on the bottom of the tank. Now while you will still get stuff in the MGF tank, it is made out of plastic, and it is located within the shell of the car, so the likelyhood of the fuel filter filling with enough crap to block it is very unlikely.
Plus, a blocked filter would not cause a mis-fire, it would cause lack of power, especially at higher rpm!
You mentioned that both lambda sensors have failed within 10 months of each other. This is perfectly natural, as if you change on sensor, you would expect the other one to have the same life expectancy. Hence changing them both, more or less together. When changing them did you use genuine Bosch or MG Rover items? As using aftermarket ones is asking for trouble!
Cap and rotor arm would be my first port of call, as these are cheap and easy to install. There is a resister within the rotor arm that can sometimes fail at lower rpm and cause the symptoms that you are experiencing. If that does not work, the coolant temp sensor as mentioned above could be causing a problem, or maybe it is a dodgy earth or loose battery terminal?
The list is endless. If you want to talk to me direct my number is 0121 5265444 i'm here 9:00am till 6:00pm (dinner between 1:00pm and 2:00pm!!), i'll try and help.