As Lewis suspected it's polished alluminium, not chrome.
It will either be laquered or clear anodized, try the Nitromorse, if that does'nt work then it's anodized (Nitromorse only works on paint or laquer), if you want to get the clear anodizing off buy some Drain unblocker from B&Q, use that to strip the anodizing then get into it with Brillo soap pads.
Lot of work doing this to a frame but will look ace once it's done, downside is it will need to be re-done to keep it looking good, either with Autosol metal polish or the Brillo pad but i've never found that much of a hassle.