I'd completely forgotten about my Stan James account,but they emailed me and told me that my account had been migrated to Unibet.The limits on my old Unibet account seem to have fallen from around ten quid to zero.No idea whether that has anything to do with the migration of my Stan James account.I tried to contact Unibet on their livechat,but got no reply.
So my plan is just to allow my existing bets with Unibet [which include a few World Cup bets] to resolve,and then to withdraw the balance.
For what it's worth,Stan James had a reputation for limiting accounts which rivalled that of Boylesports.