3rd birthday party wobbles - and any party supply places in London you can recommend?

[SarfEasticated]

My DD will be 3 on in over a week and her party is on this Saturday. Have just bought party food from Sainso - sarnies, soft finger rolls, crisps, cucumber cherry toms, low salt crisps etc, but need party plates, table cloths etc. I don't really want everything to be too themed, she does love the disney princesses, but I would rather keep that to small touches rather than the whole room awash with banners and the like. Am wondering about what else I need! We have a nice lady coming to do dancing with them for an hour of the 2 hours we have the church hall for, but I have no idea what I need to buy. streamers? balloons? banners? Helium balloons?
I have stuff for the party bags already, and the theme is kind of butterflies, ladybirds, fairy, birds.
I really don't want to overdo it, but then don't want to under do it either.
Any advice would be very appreciated, and any recommendations for where I can buy what I need in Central London, or online would be much appreciated.
Thanks

I have now been through it, and had a great time. Just in case anyone else is looking for the answers to my questions:

They hardly ate anything apart from crisps and cucumber, but I should have bought more than one carton of fruit juice each.

www.nonstoppartyshop.co.uk - branch in High St Kensington, excellent range and service.

No helium balloons, at just 3 they loved the normal kind anyway.

thanks sarf - i am busy planning my DDs 4th birthday party - will bear your points in mind Glad they had a good time!

Thanks. My DD's birthday is in 6 days, as is her party, and I've done nothing yet. Apart from the dancing, did you do party games? Which ones? Also what did you put in the gift bags?

no, had the hall for two hours, dancing took an hour, sitting down to eat food and do the cake took an hour too. They just loved running around the hall with balloons really. DD was 3 though so might not wash with older children.

party bags were:
party size playdoh - bought from amazon
terracota bird whistle from traidcraft - £2 each
chocolate ladybirds from Amazon
butterfly stickers
all wrapped up in cellophane and tied with string.
could have done without whistle or playdoh really one would have been enough

There was a loose insect/butterfly theme but really it didn't matter. i had made butterfly biscuits and decorated them with sprinkles etc (got the cutters from Natural History museum on amazon) which were lovely, but rest of food was sarnie/crisps/cucumber.

I only themed it because I liked the idea, no reason really and think other mums thought I was mad.

Cake was victoria sponge with strawberries on the top, only the parents ate it as children running around.

I laid on crisps and cheapo tiny bottles of belgian beer for parents, a nice gesture but not too alchy!

Thats all i can think of, but let me know if you need anything else!