It depends whether your former Regular military service was within the RN/RM and whether you had completed phase two training and are regarded as a fully trained re-entrant with less than 5 years gap since leaving, thus not needing re-training, apart from a phase two refresh.
It depends under what circumstances you left & whether you were recommended for further service and can still provide your last reports & pay statements. Other considerations are residency, criminality, medical suitability, whether you receive a medical pension or received a redundancy payment, your ability to pass the pre-joining fitness test, approval from your branch manager to be processed & rejoin, your financial status, providing there is no bar to security clearance & your current family circumstances.
It also depends how much former service you have had & whether, when added to the fact you must be able to complete a minimum total of 18 years pensionable service, you can achieve this before you reach the retirement age of 55.
Other than that, yep, no problem you just need to pass the selection interview.