As long as they are the modest figures you describe they should let you keep it .... after all, we rarely spend the same every month, sometimes the £50 you saved last month might be used up next month. Also, some expense categories are supposed to be saved, month on month, like car maintenance or even haircuts, until they are actually needed.
There is, or was, one firm where the IP was adamant that absolutely no savings were allowed --- but, for the life of me, I cannot remember who! This IP, thankfully, is a rare mutation of the species !
Incidentally, deposits to the bank of mattress rarely come to light.

My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014