Rainboe, you've got me there - I can't find a reference that specifies a minimum rate of descent (I'll get back to you!) However......
ICAO 8168 1.4.1.7
min rate of descent for Cat C/D/E in the final approach segment is 590 ft/min.
UK AIP AD-2 EGKK 1-15
"....a descent will have deemed to have been continuous provided that no segment of level flight longer than 2.5nm occurs below 6000' qnh and level flight is interpreted as any segment of flight having a height change of not more than 50' over a track distance of 2nm or more"
UK AIP AD-2 EGKK-1-18
"on receipt of a descent clearance the pilot will descend at a rate he judges will be best suited to the achievement of a continuous descent, the object being to join the glide path at the appropriate height for the distance without recourse to level flight"
This is fun isn't it?
Fred.