Raleigh M20 is a moderately attractive target for bike thieves
(I use that model too), so you need to be more careful with it.
Unfortunately bike lots seem to be good targets for thieves as it
provides them a legit reason to be near your bike for a longer
period of time to tamper with the lock. Cameras don't seem to deter
them that much.
Parking your bike in plain sight near a hawker centre with a lot
of human traffic will make your bike less appealing to the thief.
If possible, use a U-lock when you can... esp those without the
round key. These locks have to be picked or pried apart, so they
provide considerably more hassle to the thief.
A strong chain probably works better then those cable locks as
well, which are at best just to prevent opportunistic thief... but
a determined thief will not really be detered by them and in fact I
even accidently broke one just by pulling on it! U locks and chains
add considerable weight to your bike thou, but since u are using it
for an errand and not like going sport riding I think it's worth
the extra weight.
All the best with your future biking activities... remember no
lock will stop the most determined thief, but you can take measures
to make your bike far less appealing to them. The whole thief
situtation in Singapore is what is keeping me from getting too ex a
bike.