Ere Dave. What would you reckon to some thick plate welded to the top of an old 56lb weight?
you could do, but unless you were able to weld the full width of the plate it will make a racket and likely break apart; it's one of the problems that occurs with old anvils that have a welded on face. It does also depend on your definition of a thick plate
If the plate is very thick, then maybe use it on edge. The mistake people always make is that they think a broad surface is better, but in reality it is mass and rigidity. I've got an anvil made of mild plate that is 12" long, 8" high, but only 1 1/2" wide, I welded on a couple of bits of angle iron to bolt it down with. Not the best anvil but it has served to teach several dozed people to make axes and knives on, just needs a little redressing every year or so.
The anvils that I use in my outdoor teaching set up are made by welding a 4" length of fork lift spike to some box section that is filled with lead (though one is filled with oily sand and works pretty well), the box is then staked into a log. The sandy one only weighs about 10kg I guess and I teach people to make axes and all sorts on it. The faces are heat treated so that they take a bit more abuse.
I've got small anvils of the original rail sort of size and shape that I use for leatherwork, setting rivets and jewelry but I wouldn't ever use them when I have something more solid (irrespective of weight). 20lb sledge is great, ideally use the striking face or if you have to use it laid out then work on the ends rather than over the eye
finding a heavy and solid base is a massive bonus, be it a log or a bucket of concrete. Make sure you anchor the anvil well. I've used large bolts to hold small anvils down (having welded angle iron on to them), spikes welded to the underside and embedded into the log, chains with bolts to tension them, etc. Even one of my main workshop anvils (100kg) is a totally different beast when it is strapped down with chain and bolts compared to sitting on top of the 300+kg log buried in the ground!