Last  week, on Knoll beach, Iyle of Purbeck, I picked up a creamy coloured lump of what I thought to be a medium grain sandstone.  It appeared to have flint noodles embedded in the sandstone.  However, the rock was much denser than I would have thought for a sandstone.  Can any of you Dorset geologists give me any ideas as to its origin?

From the info given I can only give a vague speculation, but here goes!
My guess would be a septarian nodule, formed around some preexisting organic rich debris in the sediment. The core may be rich in pyrite or siderite (iron rich minerals) giving it the weight. The outer layer or cortex of the nodule would normally reflect that of the host rocks from which it came. If you can see "grain" in the lump, splitting it may reveal a fossil.

The Isle of Purbeck geology is complex but the main rocks are chalk and limestone (historically called Purbeck marble).