Based on my experience of being in the shop...depends upon who you bump into! Found them to be slightly arrogant and snobby i.e. they appear to be not very forthcoming with customer service - they don8t seek out to help you, you have to bend overbackwards to ask them with an unspoken body language (if you can8t afford it don8t come inside sort of way like that scene from Guy Ritchey's 'Lock Stock & Two Smoking Barrels' - - 'uhhh do you wanna buy a bike or not mate' i suggest you bugger off!). If you still want to Sigma, I'd recommend you ask for JIMMY - - at least you'll get better treatment than from that little arrogant Inglourious B@sterd who also works there. But Jimmy's the guy who will sort you out and look after you without your head being bitten off literally. If you bump into that little B@sterd, run like hell (believe me you'll spot him, lol) but anyone else in the shop no worries. There's no complete turn off than a bad service when you walk into a shop like Sigma that sells nothing but only the high-end stuff from BMC, Cervelo, LOOK, Colnago, Pinarello, FELT, Spesh, TIME of this world.
Others that comes high on the list;
Primera Bournemouth and The TriStore in Eastbourne (not sure where you are in the UK, but these might be a good starting point). If you are near Finchley, NW London Bike and Run have time for you and friendly even if you end up only buying small accessories or just seeking advice.
But best of the bunch in my experience DeVere Cycles south of the river in Streatham, London definitely worth a visit (talk to Paul the mechanic - top
bloke!) or the owner Maurice (ex-Pro rider!!!) who knows his bikes
inside and out. Don8t forget to have a play with their online 'bikebuilder
tool' so you can tweak various options and components to your liking
until the cows come home :) and very competitive pricing. Have fun,
iDid!