Thank you all for the great information and many apologies at my delay in replying. We've just moved onto this boat and so have been manically busy, working late into the night then falling asleep exhausted, trying to get ready to cruise for today which has all come to nothing because the winds are too high.
We're near March at the moment and will then be heading west so really unfortunately we won't be able to look for your Terrapin at Upwell which is a pity!
For info for anyone else we've been informed that the Standground Lock is being opened mornings only at the moment.