Shorties PJ set needs new elastic (Westbury Fields BS9)

Womens size 10 shortie set (loungewear or nightwear) originally from FatFace. The elastic in the shorts has gone and as it is just around the back it is harder to replace and needs more sewing skills than I have. There is a tie cord but that is only around the front. The top is a mix of 60% cotton /40% modal and the shorts are 100% cotton.