Graham, we too are insured thru IMIS via their proprietary coverage, known as the 'Jackline' policy. However, it is only available to U.S. boats and so IMIS will try to source coverage thru a range of carriers like any other brokerage. The main benefits of using IMIS is that they only represent 'A' rated carriers with whom they've had good claims coverage (to your benefit) and the knowledge they bring to insuring blue water boats, which is significant but not exclusive to them.
For European boats seeking BW coverage, many owners end up using Pantaenius (did I spell that correctly?) as their rates are reasonable and they enjoy a generally favorable repuation. You might start with them.