Hello Lindsey
I have spent some time looking into some longer term passive investments (vanguard index trackers) and investing though a fund supermarket like you mention is the only way to do if you have under £100000. This post might not answer your questions entirely though spending some time with a fund cost calculator and knowing when, where and why you are investing in that fund will help answer your questions better than I could.
The fund supermarkets which make investing in Vanguard (supposedly the cheapest index tracker fund and 'The company has a hard-won reputation for serving investors’ interests.') possible are:
http://www.hl.co.uk/http://www.sippdeal.co.uk/http://www.alliancetrustsavings.co.uk/http://www.bestinvest.co.uk/index.aspxThere are two other companies which allow you to invest in them but they charge at a rate of .5% and .85% annually which combined with inflation could reduce your investments to below the average high street bank 3% isa return.
I will update this with the costs I incur when I invest with vanguard though a quick google has taught me that in 2010 investing via Alliance Trust Savings cost per year:
£12.50 dealing fee
£5 automatic dividend reinvestment
£10 cash withdrawal charge
£30 annual ISA charge
Because of the costs I would not have though that putting money in very other month would be great for your needs, though putting in a lump sum would. The other catch to bear in mind is that to with draw from Alliance Trust Savings you have to send off a form and only once they have received it they will send you a cheque back.
I will post more information up about the costs how I set up an account and why, at the moment it looks like I am going to go for Alliance Trust as you can buy direct from the firm as long as you have a debit card and there is a £50 minimum investment per fund (Hargreaves and Lansdown require £1000 minimum). Below I have included a few links to websites with information which helped me. Let me know how you get on.
http://monevator.com/cheap-vanguard-index-funds/The best option for you might be
www.bestinvest.co.uk if you wish wish to drip feed into a fund like Vanguards as the link to the blog post below explains.
http://monevator.com/bestinvest-vanguard/Another link to a post about:
http://monevator.com/hargreaves-lansdown-vanguard-funds/Regards
Andrew H V