TFM, got this of the Aintree track site:
There is no official dress code although smart is preferable for the Grand National meeting. Ladies Day (Friday) is a spectacle of colour as many people dress up in all the Enclosures
So is it Ladies Day or the Saturday you're going to?
Is it corporate ents in the members area? Will you be doing an evening thing as well?
I see it's April 3rd so likely to still be a bit nippy. Are you a skirt or trouser girl?
If it was me, I'd go for knee length boots, A-line knee length skirt in a nice brown or green with a couple of tops layered. Actually all a bit autumnal rather than spring.
D'you like FatFace? They've got a lovely skirt in which would do the trick.
www.fatface.com/bin/venda?ex=co_wizr-productgrid&bsref=fatface&step=shopcartmulti&invt=16196&orderco l=alphanu&layout=productdetail
with
www.fatface.com/bin/venda?ex=co_wizr-productgrid&bsref=fatface&step=shopcartmulti&invt=18084&orderco l=alphanu&layout=productdetail
or
www.fatface.com/bin/venda?ex=co_wizr-productgrid&bsref=fatface&step=shopcartmulti&invt=18073&orderco l=alphanu&layout=productdetail
and, in either colour
www.fatface.com/bin/venda?ex=co_wizr-productgrid&bsref=fatface&step=shopcartmulti&invt=18118&orderco l=alphanu&layout=productdetail