Some kind poster has identified a decent map-making tool, which I'll try to use to plot my route.
Although I want to avoid the A30 in Cornwall where possible, as it's meant to be quite nasty in parts, in the extreme South West it's the only route, unless you take a very long a scenic ride round the coast. So I'll have to chance it.
On the first day I'll get to Penzance at about 1.30pm, then it's an 8 mile cycle to Land's End, where no doubt I'll be ripped off just to have a picture taken next to the fingerpost. I reckon I can do 30-40 miles that afternoon, which gets me nearly as far as Newquay.
The main problem then may be finding accommodation. Bear in mind I'll be on the North Cornwall coast, prime surfing territory, on a Saturday night in May, staying in a youth hostel. There won't be a bed. Maybe I'll ring tomorrow.
Let's see if this map works: