Standard car flasher relays are current / thermal dependant. The rate at which they flash is determined by the wattage load of the indicators. If one of the bulbs blows (or is replaced with a LED) the thing flashes at an 'alarming' rate. Alarming being the right word because it used to be considered a useful thing to let motorists know they had a blown bulb - before the days of buld failure warning circuits.
This arrangement was of course no use for anyone towing a trailer or caravan and the fix was a simple one- to use a slightly different type of flasher relay, one that wasn't dependant on bulb load but would continue to flash away at a fixed (MOT compliant) rate however many bulbs were connected to it.
These are still available and shouldnt cost £ 14 - something like this would do the job perfectly well:
http://www.ebay.co.uk/itm/Electronic...em56482 92196
Using a standard flasher relay with a parallel resistor is of course another way round but frankly not the best. The resistor will heat up quite a bit as it effectively has the whole power of the battery accross it (not something i'd want next to the wiring loom in my car, near the fuel tank etc).
Doing it the right way avoids the risk of components overheating and reduces the load on the battery and alternator.