The amount of people on this thread who think what the the OP is doing is OK is shocking.
Imagine the responses if it was a man who had squirrelled away £50,000 behind his wife's back? Depriving her of the money for 9 or 10 years.
And to the people saying those who say the OP is stealing from her husband, (and it's all so wrong,) are jealous .. get a fucking grip.
Many people on here (including me) have as much, if not more income and assets than the OP, we just don't poach multiple 10s of 1000s of pounds from our FAMILY money and stash it away.
If this tale IS true, then it is disgusting that she has kept this amount of money from her husband.
You posters clapping and cheering 'bravo' to the OP for secretly siphoning off FIFTY THOUSAND POUNDS from her family money, and accusing everyone else of being jealous of her, must be in some really poor, unstable, miserable fucking marriages. You HAVE to be if you think it's a good thing that she's done this, and that she has a 'running away fund' as you so delightfully call it!!! I actually pity you all to be honest...It can't be easy for you. 
And it seems that the OP wouldn't even have been ABLE to poach all this money from under her husband's nose if she wasn't married to him, seeing as how she was able to sneakily put her entire salary away some months.
(she said this at 23.01 yesterday.)
If this story IS true, it's disgusting. As I (and many other posters have said) imagine if the genders were reversed and it was a man who was secretly stashing multiple 10s of 1000s away from his wife?! 
And spare me the 'you sound angry' and 'you must be jealous' bollocks. Say either of those and I will just laugh at your daft remarks.
I am neither one, but I am shocked that ANYBODY thinks what the OP is doing is OK in any way, shape, or form. You can dress it up as 'it's her money too,' and 'she is entitled' blah blah blah... but it's STEALING. You are STEALING family money from your husband. If I discovered my husband was doing this, it would be game over.
And how the hell is having no sky tv a second mortgage? Daft comment!
Also I am still interested to know @LargeGnTPlease why you joined mumsnet PURELY to tell us this 'oh-so-fascinating' little story? 