The best cash rates are in internet accounts. Below are some of the best. Tax is deducted at 20% if you are a basic rate tax payer. Some accounts pay monthly, others annually. Some accounts have a limit, e.g. Alliance & Leicester limit is 25,000.
To be safe, spread the money around a bit. Compensation in the event of a bank going belly up is 100% of the first 2000 and 90% of the next 33,000.
Nationwide 5%
Alliance & Leicester 5.35%
Cahoot 5.1%
Bradford & Bingley 5.25%
Yorkshire Building Society 5.2%
ING 5%
How to invest 50,000 is a big subject depending on your current circumstances, risk profile, age etc. Put the money in the savings accounts as described above, and then go on a specialised site where you will get very detailed answers depending on your circumstances. You can then give yourself a bit of time whilst you make final decisions on how to invest your money. Meanwhile your money is earning a good rate.
I would recommend the site www.fool.co.uk . I have no financial connection with this site, just an investor like yourself.
Whatever you do, do not invest in insurance company funds etc. until you are certain it is what you want,as there can be high initial costs and they may not be suitable for your need.
All the above accounts have no notice period, so you can get immediate access to your money.
If you do not wish to become more educated financially then invest the money in the accounts described above. They pay good rates, there are no fees, and you have immediate access to your money.