Last night a Muse jumped on my head and beat this idea into it. The Thermopylae is slowww, but has a nasty bite.

Thermopylae "Heavy" Crusier
Dimensions: 5 km long, 0.9 abeam at fins approx.
Crew: 95000 crew approx.

Originally built in the small Dyson subsector during M36 the Thermopylae-class was a stop-gap cruiser inteded to stem a flow of renewed Chaos attacks. The Dyson subsector had been cut off from the rest of the Imperium by a warp storm and the fleet elements in the area were not able to stem the attacks. The Chaos forces would blockade a system and take their time stripping the worlds within. To counter this the Mechanicus and Naval leaders decided the Navy needed a vessel capable of breaking a siege or standing up to one. Power from the engines was redirected to the weapon systems allowing the batteries and lances mounted on the ship to have a greater throw weight. Extra armor was then added on to make it capable of surviving in such intense fire fights. The redesign of the weapon power systems forced the designers to move the prow weapon systems to a dorsal mount. At the time this was not considered a problem but the changes to Naval tactics have made many Navy officers consider the class obsolete. Made in M36 the end result was a vessel with the look of the older Naval style mated to modern Imperial broadsides.

Speed: 4 Maneuverability: +10
Detection: +10 Hull Integrity: 70
Armour: 22 Turret Rating: 3
Space: 75 SP: 65

Weapon Capacity: 1 Dorsal, 2 Port, 2 Starboard

Slow and Stead: The Thermopylae-class was built to be a defensive warship and exchanged speed for a stout hull. The Flank Speed extended action is a Hard (-20) Tech-use Test for this class and all tests involving a Stern Chase suffer a -5 penalty.

Rigged for War: The power from the Thermopylae-class's engines was redirected to the weapon systems for better sluging matches with the enemies of the Imperium. All Batteries and Lances installed on Thermopylae-class vessels add +1 to their listed Strength. In addition if an Archaeotech engine is installed the efficancy of such ancient Tech adds an additional +1 Damage.

Let me know what you think.