(T) HELP L reg Cinq Injector Warning Light

Newbie / 1st posting.....
L reg Cinq 899cc, 74k miles & son's 1st car bought 2 wks ago from someone we know with full MOT (car not seller). MOT had an advisory on a bandaged centre section of the exhaust (we knew about it before the guy had it mot'd). The car was previously a good runner but a couple of days
after replacing the exhaust centre section the Injector Warning light came on (and off) a couple of times with a loss of power.

I checked for loose wires / connectors, gave the injector a squirt of carb cleaner and all was ok for a couple of days then it came on again (and off) again. I noticed a leak on the servo to inlet manifold vacuum hose so I tightened the Jubille clip and again it 'seemed' to solve the problem. This time I took it for a long run and it was perfect, until a couple of days later. The air filter doesnt look too dirty but removing it (to eliminate it) made no difference.

Now the warning light comes on more frequently. I checked the Lambda Sensor using an oscilloscope and it pulses about 0.1 to 0.8 at 1 hz, apart from when the warning light is on when the Lambda gives no signal at all. The local scrappers have a few Cinqs and I was able to pick up another Lambda sensor and Fuel Injector but the car does exactly the same. I've disconnected the fuel pipe and theres plenty of flow. All earth connections are good. I've not checked the crankshaft sensor yet - anybody know where it is? We've not bought the Haynes manual yet.

At the moment the warning light is off but it could and probably will come back on at any time. Any ideas anyone the lad's doing his nut!