I have had exactly the same problem with my pixie boots, in fact although I loved them, they are possibly going to the charity shop. I wore them all last winter so assumed the suede would have stretched. You can get shoes stretched, could a shoe mender/Timpsons perhaps stretch the insteps of your boots? Maybe silk socks might help,your feet slide in rubylady or if enough of us complain to Hotter, they might redesign them with a zip. 
Can't afford a new pair, but did wonder if the extra wide might go on more easily. Alternatively they do a nice purple suede boot with 2 zips called Whisper and there is currently £10 off.