Correctamundo.
It used to be that if you had a one piece crank you just threw the chainring on. If you had a three piece then you needed the top hat washer as the axle is skinnier on a three piece. Like I said, there's a few different ones kicking about and the differences can be tiny. Sometimes very straightforward, sometimes a pain in the arse.
Measure the hole if you're able to do it accurately, chances are you just need a traditional 22/19mm top hat. Give Stodgy a ring at the RADBMX shop - he has a decent collection of various top hats. 01494 761511.