You get tax relief at your marginal rate on 80% of your investment.
If your marginal rate is 42%, this means the net cost of your investment is
25,000 - (20,000 x 0.42) = 25,000 - 8,400 = 16,600
Don't forget you are buying shares in the film. But the "return" on this investment is inevitably on the net rather than gross investment so the likelihood is the film company will refund far less than 25k to you, unless the film goes into substantial profit and has agreed to share such profit with investors.
Divinding 20k by the 42% tax rate indeed produces a figure in excess of 47,000 but this is an error in logic and there is no way your investment will produce such a return.
You really should get in touch with the accountant or advisor who helped you with this investment. He or she should have briefed you better on the workings (and expected return) on the scheme.