Since cancer cells divide uncontrolaility, then if you left a cancer cell in a flat surface, and you wait in a few days,then will the cell end up dividing a making a bit lump of cancer flesh instead?

No. cancer cells in a petri dish will grow till they have covered the whole surface and may get to 2 or even 3 cell thickness but then stop or die due to lack of nutrients and oxygen. In-vivo (in the body) they attract blood vessels which feed cancers thus allowing them to become very much larger.

So can cancers be treated by cutting off the blood-supply that feeds them? Either surgically or by a drug that inhibits the growth of blood vessels -- ideally differentially?

correct!

In theory angiogenesis inhibitors (e.g., directed to some growth factor receptors)  sound like a great idea, and a number of drugs have FDA approval in the USA or are undergoing clinical trials, but in practice there have been a number of problems. One is local (or systemic) side-effects to normal tissue, and in the last couple of decades people have been working on specifically targeting tumor cells (e.g., inhibiting/neutralising angiogenic factors) with monoclonal antibodies and/or targeted drug-delivery systems (e.g., liposomes/nanoparticles) - I don’t know how successful either has been but I doubt they have resulted in complete remission - perhaps David could comment?

VEGF inhibitors are being tested for some solid tumours but the most obvious examples are arterial embolisation of liver metastasis.