Iv just read again and it says Downloads Up to 51.7Mbs and upstream Upto 13.2Mbs Which would be great should I actually attain that speed. I wonder if there is any comback Say should I not get near these speeds?
Those speeds are only an estimate, and do not bind BT in any way to provide them. That said, they're normally a fairly good estimate and are often a bit pessimistic.
Part of the reason for the pessimism is that they allow for speeds dropping somewhat over time. FTTC speeds can be rather sensitive to the additional crosstalk as additional lines in same cable bundle as yours switch to FTTC. One additional line in unfavourable geometry to yours can, in some cases, knock several Mbit/s off your speed. BT Openreach are looking at deploying vectoring to mitigate against this kind of reduction, but there are only a couple of small trials at present.
If you're so far short of the predicted speed as to make FTTC worthless, you should be able to negotiate an exit from the FTTC contract without penalty and a return to the ADSL you have now. You should discuss this with PlusNet (or whichever FTTC ISP you select) before signing up if you want to explore your right to cancel.
With that estimate, it's a case of deciding whether to go for 40/10 or 80/20, as the additional speed of 80/20 may be relatively small. Most FTTC ISPs charge more for 80/20 because they pay more for the wholesale FTTC circuit. You have a fairly good chance of full speed if you opt for 40/10, but there's no guarantee.
With PlusNet, the choice is between 40/10 with a 40GB/month usage cap, or unlimited 80/20 for an extra £4/month. I'd say it was worth the extra £4/month for not having to worry about usage (you may surprise yourself how much you use with a faster connection) and the likelihood of getting a connection as fast as the line allows. If your phone service is with PlusNet, there's currently a £7/month discount on the unlimited offering for the first 9 months of your 18 month commitment, making the total extra cost of unlimited over the entire 18 months just £9!
I think you either quoted your speed in your first post as Kbytes/s, or you left a zero off. If you currently have the predicted 3.5Mbit/s ADSL, that likely has a upstream speed of no more than 0.5Mbit/s. Even an incredibly pessimistic 35/8 FTTC speed is ten times the downstream speed and sixteen times the upstream speed of what you likely have now.
In your position, I'd be signing up for FTTC immediately.