Have you read the Rose report?
dera.ioe.ac.uk/14790/1/00659-2009DOM-EN.pdf
Do the school do standardised reading and spelling tests termly? If not they need to. You can then calculate the ratio gain. Seems complicated at first but you can get your head around this.
The LEAs SEN policy will say what they define 'adequate' performance as. (I assume you have a copy). The Rose Review which is/was the main LA document covering dyslexia. Apologies for the long quote - according to the Rose Review (2003, pp 178-9),
'Many UK studies report results not in standard scores but in reading and spelling ages, from which ratio gains can be calculated in order to evaluate the effectiveness of the intervention. A ratio gain of 1.0 means that the child’s skills are developing at a normal pace, but they will not be catching up with their peers. Brooks (2007) suggests that ratio gains of less than 1.4 are of ‘doubtful educational significance’, between 1.4 and 2.0 of ‘modest impact’, between 2.0 and 3.0 of ‘useful impact’, between 3.0 and 4.0 of ‘substantial impact’ and above 4.0 of ‘remarkable impact’ (Brooks. 2007, p. 289).
However, Brooks (2007) points out that ordinary teaching (i.e. no intervention) does not enable children with literacy difficulties to catch up, and hence it is fair to presume that, in the absence of control or comparison groups, and where effect sizes cannot be calculated, findings of ratio gains in excess of 2.0 may be taken as good evidence in support of the method employed. Indeed, several studies have shown that, without help, dyslexic pupils progress at around only 5 months per calendar year in reading (ratio gain 0.42) and 3 months in spelling (ratio gain 0.25) (Thomson, 1990, 2001; see also Rack and Walker, 1994). Dr Singleton suggests that in cases of dyslexia the achievement of ratio gains of 1.00 or greater represents substantial progress for these individuals, even though they may still have literacy skills below levels required to access the curriculum effectively.'
I reported results to the LEA saying 'With regard to spelling, IEP data records assessment in November 2011 (nfer Nelson) of SA 7.06 at CA 10.11 (- 3 years, 6 months) whilst the post-it note recounts September 2011 (nfer Nelson) result of SA 7.05 with CA 10.09 (- 3 years, 5 months) and April 2012 (nfer Nelson) of SA 7.08 with CA 11.4 (- 3 years, 10 months). Thus in a six month period DS has made progress of around two months. This represents a ratio gain of 0.3, during the course of a Wave 3 intervention, and is clearly inadequate. This demonstrates that the intervention has been unsuccessful in improving spelling performance. This is commensurate with rate of progress prior to the specific intervention (ratio gain 0.3, July 2010-July 2011) whose effectiveness is being evaluated, and, is similar to that expected by a child receiving no help, rather than the maximum permitted at Action+ level.
Some LEAs also use ratio gains to assess the persistence of dyslexic difficulties. According to these criteria, the measure of persistence has been taken to be a ratio gain of less than 0.5 whereby with maximum SA+ support dyslexic learners are making only half the expected progress (6 months in one year). The data (ratio gain of 0.3) clearly demonstrates the persistence of learning difficulties despite well-founded intervention. Furthermore, the data clearly demonstrates the widening of measured attainment gap over time in response to appropriate and thorough learning opportunities.'
You need to work out how many months are between standardised assessments and then how much 'progress' there has been in that time. Just because some progress has taken place, it does not mean that the progress is 'adequate'.
You can do the same comparison with teacher assessed SATs Levels which you will have. LEAs have different criteria of what constitutes adequate progress (eg 1 sub-level each 2 terms or per academic year). Hence a couple of sub-levels progress over 2 years is actually regression which means that you can demonstrate 'lack of progress'. This use of language is standard 'spin' - when someone is ill we do not say that they have a lack of health!
DD should have Wave interventions (step before referral to specialist services usually although this was not offered to us until tribunal hearing). I had a meeting with an LA rep, following refusal, about our different views of whether or not progress was 'adequate', and it was agreed to measure progress during a Wave 3 intervention - standardised testing at beginning and end of one term and then calculate the ratio gain. Less than 6 months in one year was 'inadequate' according to their own policy and so they had to assess.
Simps. 
If you PM your email I will send you my initial application for SA which has charts and standard distribution curves to demonstrate progress over time (I'm a tad anal).