I think the change to AV would result in an even more undesirable governing group in Westminster, for the very reasons you outline BJ.

The phrase 'never trust a Tory' should really be amended to read 'Never trust a politician' such is the disrepute that now attaches to many of those who do eventually make it through those hallowed portals, where they appear to then succumb – quite readily - to the heady aroma of power and to the whiff of money that seems to be easily made in exchange for little effort.
The political classes are asking us (in AV) to ratify a change to the present system that will ensure the voters of this country will find it all but impossible to rid Westminster of the governing group, as deals will be done at all costs by the largest party of the group to keep power where they consider it belongs.
To my mind;
FPTP (with boundary/constituency/seat changes to achieve a more representative cross-section of potential candidates relevant to the constituency) remains the most desirable means of electing politicians to Westminster – single party in, single party out if they make a hash of things.
AV results in too much horse-trading, too many people trying to achieve their goal by comprising their principles to gain (and maintain) power, and a permanency in the ability of the main party to remain in office despite the desire by voters to say otherwise.
In short, had we been running the AV system over the past few elections we may well have remained basking under the sun of plenty that was New Labour.
Coalition governments seldom work successfully – as can be seen in what we presently have at Westminster.