Unfortunately, v4.8 is one of the complained about software versions which generally becomes very sluggish and unreliable. On the flipside, I guess your HDR255's previous owner must have received the v4.6 and v4.8 OADs, since I think they had stopped making the HDR255 model before those software versions were released. So, it's possible the previous owner returned it because they thought it was faulty, when in fact it was the v4.8 software's fault. If that's the case, then hopefully that means your HDR255 did not suffer a hardware failure at all. Of course, this does mean that you should really update it to v5.2 for best results, but by all means see how you find v4.8 first.
Your T825 had 4 aerial sockets, and physically 2 tuner units inside - the output from one tuner was fed to the input of the second tuner using a little cable, which is what my site refers to. The T835 just has single input and output sockets with the two tuners integrated on the mainboard.
So, to answer your question, it depends on your viewing habits. Yes, you can connect the RF out of your T835 to your TV as you did before, but while the T835 is in low power standby, the amplification in the tuner circuit is also off, so the RF out signal level is only a fraction of what it is normally. This means that you possibly won't get a good signal on your TV when the T835 is off. If, however, you plan to watch TV mostly on your T835 and not on your TV's internal tuner, then that's not an issue. If it is an issue, you could passively split the aerial cable before it reaches the T835 sending one output to the T835 and the other to the TV.