VW POLO 1.9CL Diesel 1997 wont start

Hi from uk check the main lead from battery+positive to the starters solenoid is secure ? also there is a thin wire that also connects to solenoid on a terminal of its own this is the solenoid energizing wire that works starter when key start position is selected if all above is ok? then use a jump lead and connect one end to battery negative post then the other end of lead to a suitable earth point on engine ? then try starting? if it works/starts ? you have a bad earth connection to engine from battery negative lead check the earth lead connection at point on engine is clean/secure ? it usually is secured by bolt at point where engine and transmission connect ? hope this helps?