Thanks jc113883, you comments set me down a path to a solution. I learnt a few interesting things along the way.
1) Trendlines can be extrapolated beyond the left and right end of the data but can not be shrunk shorter than the data series. (We wanted the trendline to represent and show only the last 6 month's data.)
2) The NA() function can be used to suppress values from displaying on the chart.
Essentially, what I did was:
1) Used the COUNTA() function to determine the total number of datapoints and hence the location of the last data points.
2) Used the LINEST() and OFFSET() functions to determine the slope and intercept parameters for the trendline describing the last 6 datapoints.
3) From the LINEST() function I generated best fit data points for the six last data values and plotted these values. Plotting outside the range of the last 6 data points was suppressed using the IF() and NA() functions. These datapoints I charted.
Regards David