Hi All. We'll, I tried Roy's method first, I used four self tapping screws as it is a big seal and tried to pull the seal out but no luck, the screws just pulled out.
I've taken the engine out of the frame today on my day off and it's now on the bench. I've taken the lower crankcase off and removed the damaged seal. The shaft was in the seal correctly and to the correct depth. The front sprocket bolts are also the correct length and the same ones that were taken off. Surprisingly the Hondabond came off really easily using white spirit.
New parts are on order from DS including the gear change seal and gaskets.
I am at a loss to understand how the seal became damaged during assembly (if that is when the damage was caused as opposed to it being a faulty seal). When I installed the seal I oiled the inner lip and carefully installed it on the shaft. I can only think the large splines where the front sprocket fits must have caught the lip although there are no sharp edges I can see or feel.
I'm going to have fun replacing the lower crankcase, holding and placing three selector forks blind. I'll dab a bit of thick grease on the bearing retaining rings to stop them falling out when I invert the case. Removing the lower case keeps the balance shafts and chain and gear clusters in the top case.
I'll keep you advised how it goes next week when the spares arrive.
Thank you all for the positive comments and advice though, all good to hear and helpful. Just a shame about the delay, this bike is for the O/H to ride now she's passed her test.