I rang up tesco over a month ago now to pay off my contract, 10 months remaining of a 24 month contract. Paid it off no problem as I have moved to another network, I didn't think to ask for a unlock code, rang them back a day or two later and they said it would take 7 to 28 days and I would recieve an email. Okay I thought, surely it can't take that long, anyway I rang last Monday to ask if they could speed up the process but they said they couldn't and that it's actually 7 to 20 to days. It was 20 days as of yesterday and I still haven't received an email.
Does anybody have any ideas as this is causing me a huge inconvenience?

iPhone 5s is the phone by the way.

iPhones don't use unlock codes. What Tesco actually does is to ask Apple to mark your phone as unlocked in their database.

You could try inserting a SIM from another network (not O2 or Giffgaff). Your phone will then contact Apple to see if the SIM is permitted, and if they've actually unlocked your phone it should work.

You might get lucky.

My new BT sim has been in since I received it but it just says invalid sim, I have restarted my phone but still no luck.

I don't know what the error message usually is with iPhones that are locked (as my only iPhone was bought unlocked).

Have you gone onto the BT website and activated the SIM? It may be that the phone is actually unlocked but your BT SIM isn't actually working.

Is the phone connected to wifi when you do this?

iPhones' as far as I recall, say something like "activation failure" if the SIM is not authorised. A new SIM causes the phone to try to reactivate.

I requested an unlock code from Tesco, was told 20 days and then I never got it. I called them after 20 days, was given the code and instructions over the phone.

Makes me wonder why the first response is 20 days if they can just do it over the phone.

you dont get a code for apple stuff - they just unlock it on the apple database - then when you put a new sim in it triggers the unlock process

Sorted it thanks to apple, got in touch via apple support on Twitter, arranged a call for today and they sorted it. All I had to do was back up my phone and do a system restore via iTunes.
Tesco mobile are useless.

That implies that Tesco had actually unlocked the iPhone, but you just had to reset it to allow the unlock to work.

I just recently bought a 2nd hand nokia from amazon, which I did not realise was locked to the EE network.

All I need to do is get an EE PAYG sim from Amazon for 1p and register it, so I can get a free unlock.

How long does it take for an EE unlock in comparison with Tesco?

EE have no obligation to unlock a phone that they didn't supply to you. They may require it to be used by you for a while before they unlock it.

Sometimes it's easier to get those guys on market stall to unlock it for you.

So who actually unlocks the phone? I have heard conflicting things.
I was waiting for an email for 20 days and it never came.

Only Apple unlock iPhones. Tesco just forgot to tell you it had happened.