'Ledgy', having many ledges, is in my 12th edition. I think 'ledgier' would follow on from that in the same way as 'busier' and 'noisier' (which aren't specifically given in Chambers).
From the Spelling Rules at the start of my Chambers...
' Adjectives ending in y preceded by a consonant generally change the 'y' to 'i' in the comparative and superlative:
happy happier happiest'.
Thanks Malone and ab. I've been away for a good while, but back on the chain gang again - I'm a bit rusty...but let's not go there with that mega-thread ;-)