So you have a GiffGaff sim in an iPhone 6,

You can get 2G (GPRS & EDGE) but not 3G or 4G

You say you have a 4G compatible sim, I'm assuming you have at least a basic Data Bundle or Tariff on the GiffGaff sim.

Does the iPhone 6 get a 3G or 4G signal with an O2 or other O2 MVNO's Sim inserted.

Sounds like a network issue, GiffGaff may need to investigate this further!

Haven't tried O2 sim in the iPhone but the android phone with the giffgaff sim in only gets Edge at this location too. Two colleagues on O2 say they can, one has an HTC the other an iPhone 5, just don't want to waste money on O2 sim, if it will get same result as O2.

Well you could always try popping into an O2 store and asking if they have any test sims or local corner shop should have a O2 PAYG sim for about a £1.

or contact giffgaff support and raise the issue with them as it sounds a weird one.

Try the O2 My Network app in the Android phone. You can get it from the Google Play store.

As someone else has already asked - are you paying for a GiffGaff 4G goodybag? You will not get 4G on your handset if you aren't. And as MrS says just because 4G is available at the location does not necessarily mean 3G is so is no indicator.

No I don't pay for 4G, and I know 3G is availableas one of my colleagues gets it at the same location, but he is on O2, I just don't understand if using another provider that uses O2s network would give me 3G when my present one doesn't.

So, can you swap sims temporarily with your colleague and try each others sims in each others phones to test 3G. That should go a long way to seeing if it is the handset or the sim. I am on giffgaff and certainly get 3G in some places but different handsets can be so variable that it could just be your handset that is struggling to get a strong enough signal.

Comparing a 4G phone/service with your 3G service is not a good comparison unless of course the person can turn the 4G off in their phone? Just checked on my Nexus and it looks like I can tell it to use 3G by preference.

Have also tried the giffgaff sim in my S4 with the same result. Tried a Tesco mobile sim and got HSPA+, but was also getting 3G on giffgaff, but this was a different car park as the usual one was full, but 3G was on and off.

Just to finish this off, I have tried a pay and go O2 sim and it seems to work, has worked all this week, any way, will report back after a longer test.

Have been using O2 for a month now, i can get even 4g now where with the giffgaff/ Tesco sims was getting just edge, so was unable to use Apple maps or Google maps, even in areas where I can only get edge, edge on the O2 sim allows me to use the above apps, so seems you can get a better service from a native sim than from mvno.