That's odd...if you were IMC you should have received your landing clearance no later than a two mile final....must have been the controllers mistake...even if the controller could not clear you to land while another a/c was in position....in IMC conditions...a tower controller has to separate and IFR departure from and IFR arrival by ensuring that the IFR departure begins his takeoff roll before the IFR arrival to that runway reaches a point no closer than 2 miles. We have to have 2 miles increasing to 3 miles within 1 minute after departure.
So...the tower controller should have cleared the a/c on the runway for takeoff at minimum while you were on a 2.5 mile final to give the a/c on the runway adequate time to begin his takeoff roll so he could have the required separation...and as soon as the tower controller cleared the a/c for takeoff on the runway, the next transmission would be to clear you to land.
We can bend this 2 mile increasing to 3 mile rule in IFR weather when we have visual on the arriving a/c...but if you didn't break out until 400 ft, there is no way we would have had visual....
seems like there were a few discrepencies with the tower guy...but who knows.