isee,
There are many well-documented projects of this sort on Gary Reysa's website
www.builditsolar.comI've built a few things with solar collectors myself. You look to be on the right track.
Putting a clear top sill probably won't do much - the angle of the sun incident on the top sill is very shallow.
If you are willing to take measurements and be "scientific" about the project, then there is a lot of benefit to building a small test and scaling up from there. Using flow and temperature measurements you can work out the amount of heat being absorbed per square foot of collector at any given time. Then figure out if the full-size collector will have a significant effect. Again, Gary Reysa does this in detail on his site, so you can learn a lot about this before you start. Also, knowing in advance if you will benefit from forced air flow (a fan) will help you design an even more effective unit (though I doubt it, England is soooo sunny).
Your shop space sounds ideal for this kind of heating project. Unlike photovoltaic cells, the shadow of a branch here or there does not hamper the entire panel's output.
Looking forward to seeing the result!