You have to put it "on retention" (fee £80) and then assign it to another vehicle, which will need to have a current MoT, there are other rules too such as the Alfa needing to have been continuously taxed or SORNed for the last 5 years, but if it's been SORNed more than 5 years, it has be taxed
Details on the
DVLA websiteOnce on retention, you can keep it there as long as you like (I think). It used to be the case that the retention certificate had a time limit and you had to pay to renew each year, but they either stopped that, or decide that after many years on retention, you may never assign it and stop charging, not sure.