Recommend a protein supplement?

There's the small issue of practicality, unfortunately.

I don't do lunges often, as I reckon I'm doing enough to my quads on the bike.

That, and it makes cycling home a bit tricky.

Yep neither did the guys I knew, and a lot of them were experienced lifters, had trained for a long time, risky exercise IMO, and I would just avoid it at all costs, front squats and jump squats are much safer and far more effective at building up quad strength, look at the quads on Olympic lifters, and those guys do endless front squats, work almost exclusively with barbells, and don't bother with lunges.