Crack in internal wall

Our bungalow had an extension built onto the lounge about 30 years ago. It's cavity wall construction, brick outer leaf, breeze block internal. A crack runs down the plasterwork where the original wall joins the extension. It's only hairline, I can't get my finger nail into it, and it doesn't seem to have got worse in the last couple of years, but does show through the wallpaper.

I wouldn't normally bother but since we're decorating I would like to sort it out. I've chopped some plaster off and can see that the new wall hasn't been bonded into the original one very well - not much better than butt jointing. I bet they didn't put reinforcing rods into the foundations either.

What's the best way to stabilise the walls? I've thought about cementing helical reinforcing rods across the joint. If I do it this way, how many rods should I use up an 8ft wall - the block couses are 9ins high? How far into the 4in cement joints would I need to cut? Would 1in be deep enough?

How would you do it?

Extensions are not tied in nowadays, and will always potentially move.

If you try and tie it in with steel rods, and the extension tries to move, it will crack even worse somewhere else

Form a proper flexible joint or put some mesh across and replaster

Could the crack have been there for more than 2 years, or is it that you have only recently noticed it? The blocks you described as breeze blocks are in fact lightweight aerated concrete blocks and they are notorious for shrinking and causing vertical cracks. If thats the case, its nothing to worry about and the crack can simply be filled.
If the crack was noticeably wider at the top than the bottom, that would suggest settlement of the extension foundation and would be cause for concern.
Is there any cracking externally?

Thanks for the advice. The crack wasn't there when we last wallpapered, 6 or 7 years ago. We first noticed it a couple of years ago when a hairline tear appeared where the wallpaper had been pulled apart. The blocks are cinder blocks, power station waste mixed with concrete, not the more modern Thermalite type of aerated concrete block.

Outside there was a crack and loose mortar where the new brickwork had been joined to the original wall. The courses hadn't been individually "toothed out" as I would have done, but each 3 courses bonded in as a block, and not done very well. I raked the loose mortar out and pointed up the joints. There has been no further sign of cracking in the external wall over the last year.

From what you've both said I would probably do more harm than good by trying to reinforce the wall, probably best to fill the crack and paper over.