I'm hoping that someone that understands the UK tax system better than I do can help explain in what way I can benefit from SEIS approved investments. I'm especially interested in understanding if there is an immediate tax benefit (in the tax year that the investment was made) from the investments, as opposed to the benefits that can be used later, when selling the ownership in the company or when it goes under.
I think this is explained fairly clearly by the HMRC page on the topic.
In particular, yes there is an immediate tax benefit in the year that you make the investment, in that it reduces your income tax bill by 50% of your investment up to £100,000 (i.e. a maximum reduction of £50,000). This reduction is capped at the actual income tax paid, so you can't directly get money back if the theoretical relief would exceed that amount.
You can also "carry back" some of an investment one tax year so you could potentially get a refund on your 2012-2013 tax bill by investing now (before April 5th 2014 when 2013-2014 ends) - similarly you could divide up an investment between the 2013-2014 and 2014-2015 tax years if you invest after April 6th 2014.
Note that you need to hold onto the shares for at least three years or the relief is withdrawn (which presumably means HMRC will demand the tax be repaid again).
You will also be exempt from capital gains tax on the shares if you hold them for the three years.
Finally there are some provisions for getting relief from capital gains tax on assets you already hold and sell if you then reinvest the money in SEIS shares.