I've re-read your OP and it seems clear to me that you ordered that particular size of bike on the advice of the Halford's salesman. He actually behaved negligently, and stupidly, by getting you to try a completely different bike, the Pinnacle, and then apparently taking a guess at what size Sirrus you need. He could have at least taken some basic measurements from you and consulted sizing guidance that Specialized presumably produce.
According to
Adviceguide, the Citizens Advice website, you are entitled to reject goods if they are not "fit for purpose". You are entitled to try the goods first as you have done, but then must stop using them and inform the trader as soon as possible if you believe the goods are not "fit for purpose".
Because you need to promptly reject good that are not fit for purpose, I would suggest you now formally write to (not just email)
Halfords explaining what has happened and saying you are exercising your right to reject the bike they have supplied you with as it is not fit for purpose (i.e. the right size).
In the meantime I think you need to take some proper advice about your legal position which I suspect is complicated by the fact you are buying through a cycle to work scheme. I believe it is actually your employer who legally owns the bike and you are just "leasing" it through the Cycle To Work scheme.
Your can get free advice from your local
Citizens Advice Bureau or from
Consumer Direct on 08454 040506.