Allocating funds for future use

[gnua at sundew.org.uk]

If I am given say some money for my birthday (say £100), I put the  
cheque into my current account.  My very kind relation who gave me the  
money wanted me to get some camera equipment.  So I've got x+100 in  
the current account now.

My question relates to working out how I can see what available cash I  
have in my account - so what x is.  It is likely more complicated than  
this, because every month I also put in 1/12 of my holiday fund.  Do I  
need to see up a liability for future spending?  I can't get my head  
around it!