Hi Andy
Glad to hear that you are off to Du Letty. It was my familiy's all-time-favourite campsite for a number of years both as tenters and later as caravanners. Like you we went to Du Letty on our first trip abroad although we had not booked in advance and just found it by searching, what a find!!!
My recommended route to Du Letty, well tried and tested at various times of the day and night solo and towing a trailer and a caravan, is as follows:
From Roscoff follow "toutes directions" signs to and then take D58 signed Morlaix.
On the outskirts of Morlaix turn west onto the N12/E50 for a short distance
Near St Seve turn south onto the D785 and keep on this road through Pleyben Christ, Brasparts (my kids loved the latter name!!!)and Pleyben
At Pleyben turn west onto the N164 until you reach the junction with N165/E60 near Port-Launay
Turn south onto the E60 and follow it south to Quimper
From the northern outskirts of Quimper follow the well signed route to Benodet
The D34 Quimpter to Benodet is a good quality road
A last tip when you reach the roundabout on the outskirts of Benodet, adjacent to the Champion supermarket, on the left, turn left, last exit on right-handed roundabout and follow tourist signs to Camping Du Letty. This route avoids the centre of Benodet which can get a bit congested at times, especially if you are towing.
The above route is a pleasant and easy one which goes through some scenic small towns, no problems there and, also, includes some good views over French countryside. The view over the distant Reservoir de St Michel from the road between Plouneour-Menez and Brasparts is especially good. I originally got this route from AA.com routefinder - it might be worth checking the road numbers as the French are in the process of changing all their road numbers!!
To whet your appetite on what you can do whilst at Du Letty I recommend you have a look at the reviews on:-
http://www.ukcampsite.co.uk/sites/reviews.asp?revid=5461
My review is number 6 and has a good list of "things to do" which we built up over the years of visiting Du Letty.
Another tip. When you arrive you will be allowed to go and choose your own pitch. Try and get one near the beach if that takes your fancy. Our best ever was when we asked and moved to a pitch right next to the beach. Our awning looked out over the lagoon spectacular.
Have a super time - just typing this brings the memories flooding back.
Best wishes
Tim