BB Lockring won't completely tighten
I had a similar issue last year. The threads on the frame to take the bottom bracket were too worn to hold the bottom bracket in place. A bike mechanic should be able to confirm this for you (for no fee). After some research I found two solutions, one of which is considerably cheaper than the other:

1: bike frame specialists / frame refurbishers can re cast the frame thread with a tough resin, and then rethread the resin, allowing a bottom bracket to sit securely. This is the expensive option.

2:You can buy bottom brackets that tighten onto them selves to stay in place. This is a sample: http://www.kingstreetcyclery.com.au/self-tightening-bottom-bracket-u118.html
However! do your research before you buy; there are different spindle lengths and other options. I replaced mine with one from ebay for about £40. After retightening a few times initially, I have had no problems since.