Citreon - Is it legal for a garage to change a date on a V5? - Potential Buyer

I've just been to look at a car at a main franchised dealer - I know that the car was taken in part exchange in April, but the date on the sellers part of the V5 (which the garage has retained) has been altered (with tip-ex) from "04" (April), to "07" (July). The garage does now acknowledge that they've had it in storage for a few weeks, but we were originally given the impression that it had just come in.

Is it legal for a garage to alter the V5 - they admit it has been changed, but can offer no expalnation!