Hi again all, and thanks for all your help!
Big news... I have a boat!
Be it fate, coincidence, or luck, I found a boat at the marina I intended on mooring at (Ely).
On our way back from Tring, visiting a boat (glad I didn't go with that one in the end), I arrived at the marina with the intention of handing over my deposit to secure my mooring, but was told they had recently got a small narrowboat that I may be interested in. So to cut a long story short, I looked, I liked, I surveyed (came back absolutely fine), and it's now mine! I am the proud owner 35ft Lady Mildred