Cranleigh, I am still having huge problems with my children's school, and I'm not giving up either with telling the school exactly what I think, or with having a good bash at getting through everything needed in order to have a good shot at 11+ and level 5s at KS2.
I don't know what makes you think that your child is average / weak in maths, but it may be something to do with the teaching your child is receiving, or the school's judgement of your child, or both put together.
Maths is a funny subject - sometimes you have to put some time into thinking of different ways the child could think about it in order to get over whatever the stumbling block is, and there isn't time in a group maths lesson for this to happen, even if the school is doing their very best.
e.g. this is example from my child - not yet 7 - new to counting up and down in halves in the car this morning. I ask "what's the difference between seven and a half and ten?" Child very confused and could not get head round it (kept on guessing either one and a half or three and a half). Then instead I did it as an interesting problem - a little girl is three and a half and she can't wait for her seventh birthday when she will have a princess party. How many years will she have to wait? Child answered correctly straightaway to this and several other similar problems. I could have hammered on with my other way of trying to explain it for hours, had tears, got nowhere, assumed child is thick. It's a mystery what goes in their heads, and exactly how they do think about maths. Primary maths is a lot about being fast at remembering number facts quickly, less on abstract concepts. There are children who are poor on the former, good on the latter, and will do much better at maths later on than a primary approach might have spotted ........ some teachers are tempted to keep child on lower level stuff until they have got that right, and big gaps develop.
You only have to get approx 50% of the Kent maths questions right to pass. Have you faced the fear and looked at the NFER sample maths papers?
What year is your child in? I don't think it's ever too late with maths to make some massive progress fast, with the right approach for the child concerned. The only hindrance is having to go to school all day each day!!
Yes if your child is truly average for the population as a whole in maths ability (rather than in what they have been taught to do so far) you will probably be hitting your head on a wall to get to the standard needed to be in the top 25% in the Kent maths paper. But if your child is above average ability wise, and is only receiving a diet of maths that will enable them to be average at maths, then that is a different story.
If you and child are keen, need to identify now what the barriers in maths are, and come up with a plan ........... me I'm still feeling flexi-schooling might be the only way forward, but it's probably pie in the sky the hope of getting permission.