They are infinite in the reverse direction, but the forward direction is a little high. I've ordered new diodes and a replacement for the transistor. We'll find out in a few days if that was it!
The diodes wouldn't cause the higher output voltage. The drop (forward voltage) across a silicon diode depends on the current through it. Look up the data sheet for the diodes you have and check the specs. I've seen a few different failure modes for rectifier diodes, basically shorted, open or leaking, and they're not subtle like too high a voltage drop. For the cost you can shotgun them easily enough but I'd be doing careful measurements and compare to the schematic to see where the real problem is.