I myself got a job with virgin only a few months after my first application was rejected at the screening stage, so it definitely can be done! I'm not sure what happens if you fail the aptitude tests, certainly for guards positions there is normally a 6 month block with them but I'm not sure if the same applies to the onboard CSA tests. Did you pass the tests and make it through to the other parts of the day?
In any case, you've got some practice now which should help you with your Chester application, so you have a positive there.
Good luck anyway, and if you do get the Chester job, you may end up feeling relieved you were turned down for Euston! Speaking as someone who's worked with the crews at both depots, I know which I'd rather be based at......