Puddytats - this all sounds very strange!
IIRC the fraud was for a cheque for £30K that passed through your DH's account? The SFO would only be involved usually for frauds where more than £1m is at stake or where there is a significant international aspect to the fraud - or other crimes may be committed with the proceeds - e/g drugs or arms.
If the SFO are involved, then I am rather surprised that the police are investigating - the SFO are not usually superficial in the depth of their evidence gathering as far as I am aware.
I'm still unsure as to why they have not been able to trace the movement of the money? Where did it go after it left your account?
In my humble view, I would sell every possession I owned and beg every penny from every person I could before I would let my dog spend a night in prison, let alone my husband. Sell the house, sell every thing that moves on ebay, beg every penny you can from relatives, move in with your parents, do everything you possibly can to get the money you need to mount a defence. I cannot imagine not doing so in your position, I really can't. Prison for the average person in the street will in all liklihood ruin the rest of their life! Avoid it at all costs.
You need to find a fraud specialist that is contracted to the LSC. They do exist! You can access advocacy services (a barrister) through LSC too. Barristers in most cases are required to represent defendants in court - solicitors only prepare the case, barristers present it, so legal aid wouldn;t work unless you could access a barrister. You can also get expert witness services on legal aid too. Of course, this is subject to capital and income tests - which you may very well not meet - it would be highly unusual for your PILs financial support to be taken into account in such calculations, I believe.
You need to get all this straight - it all sounds very very confused to me - not a good place for you to be right now! I suggest you start with a visit to the CAB, who will work out with you what your options are.