Thanks for the replies. Its reassuring to hear that immigration seem quite reasonable about it.
I was already having visions of being handcuffed, strip searched and carted out of the country in shame (im only going for 2 days with my parents as a bithday gift for my mum)!
I called the US embassy in London and they gave me an address in the US to which i could send a letter explaining the situation and some evidence to show that ive been in the UK since i left last year. Unfortunately im not the sentimental type and havent kept the original bording card but ive sent plenty of bank statements. I was worried that this wouldnt be processed in time before i go next month, but by the sounds of things, if i take a copy of the letter and evidence with me I should be able to prove on my arrival that i had left the country well within the expiry of my visa - thats if the stewardess who took my I-94 hadnt already sorted it out for me in which case i have no reason to worry about all this at all (why is nothing in life stress free and simple!).
For anyone else who has a similar problem or a visa query check out the us embassy London website as its pretty useful:
http://www.usembassy.org.uk/cons_web/ins/i94.htm
Thanks again,
rai.