Check the size of the microbore pipe, it could be 8mm or 10mm - measure the hole in the reducing piece you have.
You will need a few inches of 15mm pipe - possibly re used if someone you know has a scrap. Otherwise you'll have to buy 2 metres at the diy store along with the other bits.
You will also need.
A reducing compression coupler, 15mm x 8mm brass.
A new 15mm olive, they come in packs of 5 and look like mini wedding rings, not chunky.
Some jointing compound (water hawk or similar)
A bottle of central heating corrosion inhibitor for when you refill the system.
Tools - two adjustable spanners and a mini pipe cutter/hacksaw/junior hacksaw.
Cut a length of 15mm pipe, say about 3 inches long, but only you can see the job. Assemble this into the 15mm end of the reducing coupler using a smear of jointing compound on the olive. If you're not sure how to do it, read up first.
Then assemble the other end of the 15mm into the inlet of the rad valve in the same manner. You will now have an 8mm socket pointing downward and a bit of 8mm pipe that is too long. Cut the 8mm pipe to the correct length, bearing in mind
that it has to slide into the socket so may need to be longer than you expect.
Assemble the 8mm joint in the same way as the other two, using less force, it's smaller - don't overtighten.
Fill the system, check for leaks and fix any that you find.
Once you're sure there are no leaks, add the inhibitor to the system water (have a look in the FAQ's). Inhibitor is essential, don't skimp on it!
It won't be the prettiest repair in the world but it will work if done correctly.
Hope that helps.
PS here is a pic of a reducing coupler -
screwfix don't seem to sell 8mm ones but it will help you know what you are looking at -->
http://www.screwfix.com/prods/51238...ng-15-10mm;jsessionid=T1E3KHVFVX4LCCSTHZPCFFQ
And one more tip, diy sheds like
wickes and b+q are more likely to have what you need than homebase/focus etc.