Childcare vouchers taken from SMP?

Hello,

Wondered if i have been paid correctly.

My work offers 3 months full pay and 6 months SMP.

My maternity started 17th Decemeber my full pay is upto the 17th March.

Im in salary sacrifice so Childcare vouchers of £243pm are also deducted. I am paid 28th of each month. Jan 28th and Feb 28th I had the full £243 vouchers deducted from my salary which Im happy with.

But......March 28th pay I recieved 2 weeks full pay (upto 17th March) and 2 weeks SMP, plus an annual bonus.

Childcare vouchers of £243 was also deducted from this. My question is should the FULL £243 have been deducted if I didnt recieved full salary?

My understanding is when on SMP childcare vouchers become a benefit in kind so cannot be deducted from SMP?
So I should only have been deducted the cost of the vouchers upto the 17th, not the full month? Am I right or wrong?

I have tried contacting work but they are rubbish at getting back to me.

Thanks

Laura