If you use a "Qualpex" barrier pipe or similar you can keep joints to a minimum. If using compression fittings with pipe support insert some plumbers will use a smear of jointing compound (boss white ) to ensure a leak free job. However there are new technologies available which make plumbing easy for the competent DIYer, such as Polyplumb, Hepworth, Acorn etc. These pipe systems use plastic push-fit fittings which require no special tools for manipulation. They cut down on labour and are almost guaranteed leak free, provided you follow the manufacturer's instructions. I'd be inclined to use plastic if burying in concrete, direct contact between copper/ brass and concrete is not recommended as corossion can result over time. However any pipework within 1m or so of a hot water storage vessel or boiler must be copper, no plastic allowed for obvious reasons. Also where making connections with your rads I'd use an elbow and run the feed and return tails in copper, looks neater and more professional.