Harry,
I'm very sorry to hear that you have been ripped off, there really are some right sods out there. The problem is you are now stuck with them, a bank will just confiscate them. Sadly there is just no way round this.
Given the sheer volume of notes that pass over the desk, it would be impractical to and time consuming to check each and every one. I have a little experience in detecting forged and counterfeit documents, so here are a couple of tips for the future, I'm so sorry it's too late this year:
* Try and ensure the watermark is genuine by holding the note up to the light. This may sound obvious but counterfeit watermarks are often printed in a light greyscale. Genuine watermarks are designs simply embossed into the paper and are created by the variable thickness of the paper. No ink is involved in their creation.
* Watermarks for this reason should never, ever, fluoresce under ultra-violet. This particular device is highly recommended for a hand-held UV scan. It's only £10 inc p&p:
http://www.c-p-p.co.uk/product/asp/ProdID/2080/CtgID/2287/af/page.htm
The lamp also includes a point light source that one can use to back illuminate watermarks to ensure they are genuine.
* The Queen's hair and crown should be composed of intaglio 'ever wet' ink. This means that if you run your fingernail over it it will feel scratchy and will rub off if rubbed against a piece of white paper.
* Remember that cheap quality paper often glows bright under UV, unlike proper notes which should have a very dull reaction, aside from the printed UV safeguards. In other words, the note should not glow, aside from specially printed designs.
* Ensure that the hologram does not scratch off with your fingernail.
* Try a tiny tear along the silver security strip. Counterfeits tear through, genuine ones should not rip at all.
* Print quality should be nice and defned all over. Beware dots in background printing, that belies ink-jet home printing.
* Try and familiarise yourself with an area of a real note using a magnifier such as the following cheap 10x stamp loupe. Beloved of stamp collectors, it's also very useful for detecting forgeries:
http://www.maplin.co.uk/Module.aspx?ModuleNo=32331
Once familiar with a real note, false notes should be remarkably easy to spot. I'm under no delusions that checking each and every £20 note would be impossible. However, if one has the slightest suspicion or something feels 'odd', one should have no compunction on giving it a ten second once over using the above tools. Ultimately it's better to be safe than sorry even if it holds a punter up briefly.
If you do get the above tools, I'd be more than happy to pop up the mill to give you a demonstration.
Regards!