OP seems to have deleted the OP now, I won't be specific if he now wants that info removed but what he did was minor and wouldn't be found out in a regular company and/or is pretty minor and likely could be explained away if it were discovered.
The problem is that in financial services (indeed in anything government related requiring vetting) you have to be completely transparent, even a small lie or omission on a CV can be found out and cause for failing the check.
Just for the benefit of anyone reading who hasn't worked in finance, the checks I've had tend to ask for 10 years of employment history AND for you to account for any gaps, the checks get carried out by a private company and they will check everything you put down, you need to account for every month over the prior 10 years, they don't care if you weren't employed for X months at some point, they're not the hiring manager/you're not trying to impress them by jazzing anything up, they just want dates/confirmation that you were at X employer, X educational institution or doing whatever it is you put down etc.. at the dates you claim, that's it + a credit check & criminal record check.