"...and ive just booked a nice holiday in mexico....im after a rucksack of some sort preferably something around 40 litres ish for around 40 or 50 quid? Seen the dd bergen, alsohighlander 44 and the the mil tec tactical bags..."
Are you taking the pack on holiday to Mexico with you? If so, buy something used of Ebay, battered but not so abused that the seams will give way. Do not buy anything too military looking, do not buy anything that sets your pack apart from any of the others that folks on buses, trains, hotels and hostels might have.
You need something that fits you and is comfy, so try out a few brands at the local climbing/camping shop to get an idea of how they sit on your back.
Do not dwell too much on 'which pack' or 'which thingy' it is entirely possible that your pack and all of its contents may be stolen or lost either in or on the way to and from your destination. If that happens it doesn't really matter, you can pick up a 40l rucksack locally for next to nothing, or you could throw something together with a blanket and some string.
The most important things to get home are you, the contents of your camera SD cards and any notebooks you might have with you. Keep them on your person, everything else is replaceable and their loss will be just another story to tell.
Have fun and post some photos here on your return.