Hi June
I have had a look at 1851 for a Matthew Thomas born Llandyssil, and about 55 years of age. I can find no Matthew that fits with the details.
The only one I found was aged 61, living in Welshpool but the place of birth is shown as Worthen, Salop.
As you have already found him in 1861 in Shropshire, it may well be that he was in Shropshire in 1851 also. Post a request on the Shropshire Look up Board.
I do not have access to the 1841 Montgomeryshire census, so it might be worth you posting a request for that on the Montgomeryshre Look up Board. I do know that at least one Rootschat member has the 1841.
When posting your additional requests it would be worthwhile including all the infomation that you have found about Matthew so far -
year and place of birth, his wife's name, and names of the children. You should also include any other census details that you have found the family in.
I hope this helps.
Jan