Hi Jack Pinfold !
The Engine Management Light ( which Triumph refer to as a Malfunction Indicator Light, or "MIL" )
comes on whenever a Fault Code ( which Triumph call a Diagnostic Trouble Code, or "MIL" )
is stored in the ECU.
It is quite common for DTC's to be generated and stored when Battery Voltages get low.
( "P1698 - 5 volt sensor supply malfunction" crops up quite often for this ! )
The MIL will be extinguished after a certain number of "No Fault" starts, typically 3,
and the DTC will be erased from the ECU Memory after a certain number of "No Fault"starts -
typically 40, making it important, especially when intermittent faults might be involved,
to check for DTC's sooner rather than later, and record them for later reference if needed.
( Photographs are a good way of doing this ! )
Have a look at this Topic, where you will be able to find out HOW to display any stored DTC's on your
Instrument Panel, and where you can download a Document listing all of the DTC's and their meanings :
Servicing Data, Reference Info. & DTC ( Fault ) Codes -
Scroll down to Reply #5 dated 11 March 2015,
titled,
Diagnostic Trouble Codes ( DTC's ) / Fault Codes 
Reply back here with any DTC's that you have for further advice.

Meanwhile it would probably be a good idea to get your Battery professionally tested,
to check the Battery terminal connections are clean and tight, and also check inside
the Main Fuse ( the one on top or adjacent to the Main Battery )
for any signs of corrosion that could cause a poor contact.

Cheers

