Basically I was ready to hammer on the race. However it slid right down to the base. Is it classed as too worn to use or will the headset bolt create enough torque anyway?

(Steerer is one thickness the whole way down. Not belled/thicker towards base)

(Editted)

be fine

you coudl put a bit of threadlock under it if you're worried.

just tried the race on another fork- its harder to get on-bottom of the steerer 'bells out'/thickens.
On the steerer of this fork though its more 'straight'. Race is a Chris King. Fork that thickens at the base of the steerer is a Pike and the steerer that is the same thickness the whole way down is a Lyrik.

if it is able to spin freely then discard and source another race, bit if it fits all snuggerly then run it as is.

FACT

It'll be fine
I have one that I can install & remove the crown race with just my fingers

I think CK can supply undersize crown races.
Alternatively you could try one of the various strengths of Loctite retaining compounds such as "Bearing Fit"

Can steerers sometimes come in one thickness though? (secondhand forks)

the tolerances should be quite tight between steerers. if you are worried, have the crown faced then fit the headset and check for play.

Eurika. I've just found another CK race. Tight when it reaches the last 1/4 inch. Its a very worn race. although wierdly the Pike forks have a very visible 'step' for the last part of the steerer.

can vary if the steerer is steel or alloy, old bean

Both are alloy though. Sniff, I dont buy lower end models.