The questions I have in relation to this bench specifically are:
1. Am I going overboard having 4 legs at the front, given the size of the bench would 3 be enough?
Two would be sufficient if they were strong enough. The more legs you have, the more contact points with the floor that need to be spot on. If one of those legs doesn't make good and even contact, then it's not doing anything...
I'd suggest a cross brace near the bottom though.
2. Even though this will be fixed to the wall should I put legs at the back of the bench as well, I don't think legs at the back would make any difference in terms of strength and stability which is why I removed them.
Nope, legs at the back won't do anything if the wall fixings are strong enough. You can always add one or two supports at the back if you feel it's needed though.
3. Given that I plan to use this for metal work and engineering, is it worth considering a granite worktop instead of the top sheet of plywood. I ask this because I've seen ads for second hand granite worktops at quite large sizes for almost the same as the cost of the plywood.
Sure, if you aren't likley to chip it by smacking it with a hammer
Fold up workbench aka Diana: This is going to be my main woodworking bench and will have two 9" Eclipse vices (front and tail), based on the recommendation of Paul Sellers. Although I'll be honest, I'm putting the tail vice on more as an indulgence than a necessity. Expensive indulgence I know. I'm mounting this one to the wall using 3 brackets which have a weight rating of over 500kg.
The top will be 1400 x 700 mm and 50 mm thick. Like Bertha, this will have a sandwich top, but I'll be using two sheets of 12mm instead of 18 mm. Everything else will be the same 4 x 2 redwood. I'm planning to use 3 legs with a cross beam at the bottom at the front and they will be fixed to the frame using hinges, so they will fold in when the bench is in its folded position. Not sure if 3 legs are overkill in all honesty.
As above, two would be fine
Mini Workbench aka Tara:
The bench top will measure 800 x 400 mm and be 50mm thick, I'm actually undecided on whether I want to stick with the same sandwich type top or just go for a 50mm thick laminated top using redwood. The legs will have feet that allow for clamping and aprons which will fix it to the top of the bench. With this being a smaller bench I'm going to be using 2 x 1 redwood for the legs and aprons and the feet will be from 1" blocks. One of the concerns I have with this bench is the length as it is longer than the width of my other benches and I'm just wondering whether that will annoy me.
Not sure why you think this 800mm bench is longer than your 2000mm and 1400mm benches? Am I missing something? Also not sure why you need to complicate things by having it slot over the other benches. Why not just have feet that allow you to clamp it to the top of a bench with a couple of F clamps - then you can have the mini bench whatever size you want. Laura Kampf did a nice mini workbench on YouTube
Firstly is redwood or whatever softwood I get from the local timber place good for using as a bench or would it be better to use a hardwood which would comfortably increase the cost 5 fold if not more. Given that I intend to sandwich the top with plywood I'm fairly certain softwood would be fine.
It's a matter of personal preference, but the consensus on here is that softwood would be absolutely fine.
My biggest question is whether the sandwich type bench top is worth doing. My reasoning is that the first layer of plywood will be easier to screw into the box frame and provide a flat surface for the middle, the laminated western red cedar will give stability and strength and the top layer of plywood will give a flatter surface to work on.
My concern would be that the top ply veneer of plywood is often much more fragile than normal timber. It's really easy to take a chunk out of a piece of plywood, whereas you're more likely to just dent timber.
In terms of the Box Frame for Bertha and Diana, would it make any difference if I used 2 x 1 or 2 x 2 as opposed 4 x 2 for the frame.
Yes of course - the bigger and beefier the frame is, the more stable it will be. I'd personally be recommending you use a minimum of 2 if not 3 laminated 2 x 4 for each component - have a look at my workbench thread here for an example prototype-workbench-build-wip-t109850.html
I think beefing up your frames should be the number one priority if you want solid benches