Some jobs have a reserve list for 6 months, some have it for 12.
I was put on a 6month reserve for an EO role and within hat time got 2 offers. It is possible to have it extended, but not always the case, it might depend on location and role and whether there is usually much turnover of staff.
Check the email you got in March about the reserve list. I had applied for a role in DWP and any emails they send they include in the signature contact info and my reference number if I have any questions