Thanks everyone for your input so far.

Everyone's comments so far are suggesting that the short life of my
LEDs is caused by over-current/spikes.  A constant current supply has
been recommended by a few people so I duly went to wikipedia the great
source of information and mis-information to get some ideas in that
vein. Before I ask for advice on how to implement this idea I should
provide some more information on the set up as it is, and why I put it
together that way.

The LEDs are powered from the side-lights circuit on the bike by
splicing the line coming in just under the headlight assembly.  This
means they are only powered when the ignition is on and they can be
switched on and off using the normal handlebar switch along with the
other lights.
There are 2 each of white and yellow LED strings each with 24LEDs,
24cm long mounted on the sides.  There are also 3 strings of 12 white
LEDs mounted around the top.
The top strings are driven directly from the light supply and the
white and yellow side strings are driven via a pair of relays.  When
the turning indicator is inactive, the white string is lit (normal).
When the turning indicator is lit, the yellow string is lit and the
white string is not.
I set it up this way because I thought having lots of white lights
facing forward and very little area of indicator visible from the
front, it might be difficult for oncoming traffic to see when I'm
indicating to turn.
If anyone has a better idea than the relays for the indicators part
I'd be glad to hear it, by the way.

Providing a single constant-current source for the whole set up might
be problematic when indicating as I expect my relay will have a breif
moment of not powering either of its connected 24LED strings meaning
the current source(tuned for normally driving 84 LEDs) would be shared
between 60 instead.  Is this something I should be concerned by?
  If so, perhaps I should have 3 separate constant-current sources.
One for the top 36 LEDs and one each for the side sets with their
relays.  The next thing I need to know is what's the correct constant
current.  I plan to determine this by measuring the current drawn
using a clean 12V supply prior to re-installation.

I'll also need to build and weatherproof the new circuit(s) and
preferably have them all together in as compact a package as possible
along with the relays.  This will be tucked into the space just under
the headlight and secured with cable-ties. This package would have 10
connections, 2 for the incoming supply, 2 for top LEDs and 3(x2) for
white/yellow/ground on each side. I plan to use the standard 2 and 3
way connectors similar to those found all over the rest of the bike.
--=20
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