Two points - first, where would the ditch drain to? Is the next bit downhill still open? What about the bit uphill from you? If your bit is filled in, is the uphill ditch water draining onto your land and adding to the problem?
Second - you need to speak to the ROADS people. It is likely that it will not be a problem but they are the ones to make the assessment. Someone is bound to clipe if you don't approach them first

When we did a small amount of adjustment to a drain by the side of our tiny unclassified road the Roads men were there pretty promptly to see what we were up to. They approved what we were doing, but we were informed that the verge is definitely the councils and the roads men have responsibility for it.
There's a glib little phrase going round at the moment - "It's better to ask for forgiveness than permission" but I don't agree - it's best to do things the right way first time or it always comes back and bites you in the bum
