Depends on which exchange in Hertford you're connected to, and further from there which DP (distribution point, the green boxes) you're connected to as well. The upgrades are done at the green box so different areas will get fibre at different times, even where people are connected to the same exchange.
Your best bet is to put your phone number into the checker at http://www.dslchecker.bt.com/adsl/adslchecker.welcome
- if it offers you an estimated speed for FTTC then you'll be able to place an order for it.
Hope that helps explain.