I think that's what it's called, the hose from the Rocker Cover that does down to mid-engine?

Mine is split but it also appears to be far too big in diameter. It was a very loose fit and the jubillee clips seem to be near half the O/D of the hose when tightened. I have measured the fitting and it seems to be 3/4" so I was just going to get some new 3/4" hose to replace it.

Is there any reason not to do that, should it be oversized and if so, why?

You will probably find it just swollen up over the years with oil contamination, just get the proper hose its probably cheap as chips.
5 quid delivered http://www.ebay.co.uk/itm/Land-Rove..._CarParts_SM&hash=item4aad468bf8#ht_864wt_907

Lynall

It was extremely soft so I guess it was originally the correct size and swollen. I had some 3/4" hose so I have replaced it, it was fine with straight hose, didn't cause it to kink.