Thanks everyone for your advice. I contacted Google again but received two contradictory replies to the same email:
Email Number 1
"How to check if a device is on LTE or not:
1. Signal strength indicator: When the device is not connected to Wi-Fi, the signal bars at the top of the screen will indicate the network type.
LTE: Indicates an LTE connection
H: Indicates HSPA or HSPA+ connection
3G: Indicates a 3G connection
E: Indicates an EDGE connection
2. Settings: On the device, open Settings. Navigate to About phone > Status > Mobile network type
If the device works fine on other network types (H or 3G) but not on LTE, please contact your network operator for support. If customer is in an LTE area like New York City and tries to connect in Maine, it may not work if Maine doesn't have LTE with Verizon."
Email Number 2
"Thanks for getting back to me about your inquiry about 4G availability on your Nexus 5 in Madrid, Spain. On your Nexus 5 device the service strength indicator symbol will pop up as "H" when you are receiving a 4G signal. As discussed earlier the 'H" stands for (High Speed Packet Access), so this symbol will be your indicator. Your carrier Movistar is also a good source of information and any updates on their Network coverage and availability."
Seems like a bit of a contradiction to me..
The problem is I should have 4G but "only" get the "H" symbol and I am in a supposed 4G area, with a 4G contract and a 4G card (and a 4G phone!). Speeds are faster than 3G but well under 10mb/s. Without knowing for sure what to look for on the phone I don't quite know what to say to the carrier (assuming this is their fault!).
The carriers in my country of residence (Spain) only claim to offer 4G rather than 4G LTE, to complicate matters.
Thanks