Nick,
Whilst the issue may seem petty, the advice is very sound, and a variation to the deed should be neither difficult nor time-consuming for solicitors to sort out, and would give you the right to access his land to lay the drain and, in the future, to maintain or repair it. You also mention about him connecting into it. I would suggest then that it is vital to have the various rights and responsibilities set out. Otherwise, it could go horribly wrong at some point in the future - for example, you may be faced with the drain being blocked by his actions and you having to fix it, or worse still , its blocked up for you, but flows for him and he won't let you on the land to fix it. Finally, it could also affect matters in the same way with a potential purchaser, as and when you might come to sell your place.
MG Mark