Hi Adrianne,
Sanvey gate is from Old English sand-weg "the sandy way" + Old Norse Gata "road"
In 1316 this road was known as le Sckeyth a name that derives from the Old Norse Sketh "a track,a race,a race course".
info from; Understanding Leicestershire & Rutland Place Names by Jill Bourne.
It is very possible that in 1861 that Sanvey Gate was still pronounced in Leicester dialect as Sandygate.
Hope this helps,
Mark.