Started a new job, but going to uni, how do I tell them?

I applied for uni in September with every intention of going, but needed something to do for the year so also applied for a job which looked pretty good. I got the job, as a permanent position, but didn't mention uni because I assumed it would be fine leaving after 11-12 months.
Fast forward to January, and I had an offer from Cambridge, which I will obviously take.
They began the pre-employment checks as soon as I got the job but subsequent delays meant they weren't complete until end of January. I was tempted to tell them about uni, but felt as they had gone to the effort of doing all of the checks, I should at least work for some time.
Now working, and not particularly enjoying it, it's not what it was sold to be in the advert, and know I will be leaving in a few months anyway for uni, yet haven't mentioned it at all to them.
When should I tell them, should I just leave it to my latest notice period and tell them early summer, even though I'm not really enjoying it? Or do I just let them know now, and just work until they find a replacement?

Advice would be really appreciated.