The plant and equipment currently cost 800k. In four years time, with inflation, they'll cost

.
Meanwhile, the 200k currently in the fund will be worth
after 4 years. So at that time Purdie Ltd will need to have saved an additional
865,945.72-259,231.44 = 606,714.28
pounds in order to pay for the new plant and equipment.
So now you simply need to calculate how to raise that sum in four easy payments:
This assumes that each payment is made at the end of the year. In other words, the first deposit of that amount is one year from today, and the last deposit is made four years from today - the day that the new plant will be purchased.