In the same way that SMP is based on money received during weeks 17-25 of pregnancy, many employers will use the same to calculate their enhanced maternity pay as well, or may specify that it is the amount you are receiving at the date you start maternity leave, meaning if you are already on maternity leave from a previous baby, you wouldn't get it.
I'm just speculating and giving examples of things to check for. If your employer uses terms like "full pay" and "half pay" in their maternity policy without defining what those terms mean, then you are reasonable to assume that is based on your contractual salary, even if you haven't actually received that amount for a while.
Just check carefully the conditions your employer put on their enhanced pay and how they calculate it, basically.
There doesn't have to be any gap at all to qualify for maternity leave. However the issue comes with how pay is calculated. Statutory Maternity Pay is calculated using average pay in weeks 17-25 of your pregnancy so if you're not back at work for at least some of that time you may not earn enough to qualify.
If your employer offers enhanced maternity pay they set the conditions so you'll need to check their policy to find out how it's calculated and whether there are any in between leave return to work requirements.
In my current role, it states that to qualify for Maternity Leave you have to have 26 consecutive weeks of service at the 15th week before your due date to qualify for it.
What happens if you just had a maternity leave (1 year) which included some months without any paid leave in that time frame. How soon after can you become pregnant and qualify for a second maternity leave?
In general I have started working for this company more than four years ago.