I thought I'd give an update in case anyone else with similar issues reads this thread.

I think my problem turned out to be muscular, rather than because of the catheter. I asked my FS for a speculum and set to work practising slowly opening it further and further as my confidence rose. As a result I had the most painless transfer this week...absolutely none. I think a papsmear would have hurt more. If I need further transfers I will practice with the speculum the week or so before hand.

GL to few of us ladies who find speculums difficult.