I've got a Nokia 3310 on Virgin pay and go still lying about, can I put that into my new contract Orange Nokia 8310?

If all the opitons on your Orange Nokia are working you should be able to swap over the sim cards and put your virgin pay as you go sim card in, but you might have to go into the menu that selects the different networks and select the one that virgin uses.

It depends if your Orange Contract phone is SIM-locked, ie. locked for the Orange network only. I think Orange and One to One SIM-lock all their phones so that you can't use them for other networks. There is however no harm in putting the SIM card in and see if it works. According to the following website which I found using Google, Orange will unlock a contract phone for £30:

http://www.1st4nokia.com/unlock.htm

The website can do it at a lower price but doing it may invalidate your warranty. So proceed with caution.

All the best

H'mm - I phoned Orange CS on exactly this issue, and they told me that to unlock a phone that's still under contract, you have to pay the entire outstanding contract-rental upfront (eg 12 x £12.99 if you've only just taken an Everytime 20 contract), and then pay a £20 admin fee on top of that.
Of course, you're then not charged any line rental for the rest of your contract, having paid it all up-front, and you just get billed for the calls.

Oh yes, and you also have to had had the contract for more than 14 days (probably due to their returns policy)

Presumably if your contract has expired then they just charge the £20 admin fee.

the best way to unlock your fone is to take it to a small local mobile phone shop and ask. they charge 5-10 quid!

Or of course, get a data cable and download the software to do it yourself