BES schemes are high risk in nature. You could end up losing all of the investment.
Regarding the investment - [broken link removed] may be of use. From my brief reading of it if you invested 20k you would be entitled to tax relief of €20,000 * your marginal rate of tax. On the basis of your figures it looks like you are paying the 20% rate and there fore for your investment of €20,000 you may get relief of €4,000. If you wish to reduce your bill by €20,000 you would have to invest the €100,000 (and remember you could quite easily lose all of the investment!).
Pay close attention to all the criteria in the link and if thinking of doing this speak to an accounant or a tax advisor before you do anything (and probably also an investment advisor!).