You can buy say a SGS2 @ £450 and an iPhone 4 @ £450 and try and resell them, the iPhone will get at least £400 the SGS2 will be lucky to reach £300-£330, the SGS2 is in the same resale bracket as a 3GS

Because they get outdated far quicker than iPhones do. Where an iPhone will be the 'latest' for almost a year, an Android phone is lucky to be the latest for a few weeks...

The SGS2 is about to be outdated by the new generation of dual core Android phones...Nexus Prime, HTC Pyramid, SGS2+, LG Optimus...so the value of the current SGS2 is going down as a result...

It's just the downside of such a fast moving market...

Looking at past sales on eBay you can get new iphone 4 for £399, used sell for £250 - £300+

3Gs you can pick up for £100 - £150+

if you look at the actual completed auction prices of iphone 4, they appear to be around 300-350. People might ASK for £400+ on ebay, but its rarely paid, unless its some sort of special edition.

paid 100 quid for my Orange San Francisco in October 2010 and sold it for 104.56 last month. Question is why are Apple phones so poor at holding their value?

Good point, I did similar selling my San Fran to some guy in Russia, I suppose it just all depends on the phone.

I bought a hand of bananas for £1 and sold it for £2 so the question is why are phones so poor at holding their value?

We are talking the standard price people pay for a SIM free handset, not a lucky deal you may or may not happen to have got.

This is exactly the reason.

Because few pay full price and the Android marketplace is competitive.
I'm finally upgrading my phone with a 16GB memory card. Justifiable at £11.

If you look around you will see that on contract even the Galaxy S2 sells towards half the price of the iPhone. Headline prices can also be halved or more for those paying cash on PAYG.
Competition is always good for the buyer which Android certainly provides.

Of course if someone is daft enough to pay £450 for an old phone then there's plenty daft enough to pay £300 for it 2nd hand.
Those on Android can pay as little as £140 for something more powerful so why would they even want the risk of something 2nd hand?.

Don't Apple also price fix and take a cut from networks price plans? So there's no chance of subsidised handsets or cheaper price plans so keeping their value artificially high - I've quite often seen the latest £499 Android phone on £20-£25pm contracts with free handsets and more minutes/texts/data wheras iPhones tend to be £30+pm and a couple of hundred for the handset for poorer deals.

Absolutely, different models can cover a wide spectrum of user affordability.