Re-pointing cracked brick lintels

[cycleman]

I'm re-pointing the front of my house but I'm a bit concerened about cracks in a curved arch brick lintel above a upvc window. The cracks are in the centre, and down each end, and they show on the internal plaster, so they go right through. There is a very small amount of settlement visible in the centre - either side of the crack the bricks slightly mis-align. The mortar is lime based (soft), but the pointing I'm removing is hard (cement based). I'm worried that removing this hard stuff will lead to more settlement, even collapse, at worst.
I'm sure a builder would tell me that all is well, the arch is self-supporting and the cracks only occerred when the windows were replaced and will go no further - but I'm not sure about this. Can anyone offer any advice? Or am I just worrying about nothing? Your opinions, suggestions, etc, would be much appreciated. Thanks (please see picture in 'my album')

 

[^woody^]

Lime mortar is always going to be flexible, and so you are probably seeing cracking in the less flexible cement mortar and plaster.

The arch may well have been upset by the plastic windows, but more likely, it looks like there is daylight between whatever that black thing is above the arch and the wall

So it may be that water is concentrating (running down from above) in that area causing drying and wetting (shrinkage and expansion) of the lime mortar. This in itself is not a problem as the arch should stay up, but it wont help the cracking

 

[cycleman]

Thanks woody. Sorry about the misleading picture. That line of 'light' is actually the sun catching the bottom edge of the fascia board. There is a black gutter, a black fascia and soffit board - but the evening sun is casting such a strong shadow on the wall it just blanks out everything. I have started the job, but not raked out the joints on the lintel yet - been practising on the wall, and so far all seems to be going well. Also I have made a curved former which is braced off the internal sill with wedges, which gives me a bit more confidence to start removing pointing from the lintel outside. also I will probably grind it out with a diamond blade rather than using a chisel. What is interesting is how sunlight has affected the strength of the old pointing. Areas of old pointing in permanent shade are rock hard, those exposed to sunlight - crumbling.
All the best and thanks for your reply