£50-£100/month to invest....

Scully's_Girl

Definately go with an ISA first. If you are taxed at 20% chances are pretty low that you would get a return on your investment (any investment) of that much.
With £100 you can start with a cash or stocks/shares ISA. There are pros & cons to both. The cash option of £3000 max/year will guarantee a return of X%, but it will never go above that. A stocks/shares ISA can potentially return more, or less. Ah, theres the rub. Check out www.fundsupermarket.com to see some options. I had a cash ISA but realised I wanted more return, so I went to a stocks/shares last year & the return has been very positive...so far.
If you want return you have to take risk, thats the long & short of it. Debt_Free_Chick is exactly correct, when you find a fund tell the company you want an ISA first. You are investing in the markets but are sheltered from tax on the gains. I'd take that first any day.