Hi Dan and welcome to Rootschat.
I am assuming your details relating to "Great Casterton-Cum-Pickworth" are from familysearch because Great Casterton and Pickworth are actually separate parishes and although various online sources suggest that the church at Pickworth was destroyed in the 1700s, separate parish registers for both were maintained. The IGI has clumped them together, possibly because they only have the Bishops Transcripts which appear to have been combined for both parishes.
Apart from the burial of Frances Merriman dau of Edward and Elizabeth which is in Great Casterton in 1762, the Merriman's were in Pickworth for at least two generations but it looks like they weren't from there originally.
Edward Merriman was baptised 14 Feb 1713/14 to Richard Merriman at Pickworth, no mother mentioned. However his wife would appear to be Frances because of other baptisms:
4 Feb 1711/12 Frances Merriman dau of Richard and Frances
4 Apr 1716 Robert Merriman son of Richard and Frances
6 May 1718 Thomas Merriman son of Richard and Frances
All baptised at Pickworth.
I can't find a marriage for Richard and Frances in Rutland, so it may have been over the border in Lincolnshire. Frances was buried in Pickworth on 24 Aug 1766 aged 82 years, can't see a burial for Richard at present.
Richard was baptised in Pickworth too:
3 Feb 1682/3 Richard Merriman son of Richard and Elizabeth
Richard Merriman married Elizabeth Wing at Pickworth 5 May 1676.
Richard snr was buried at Pickworth 29 Jun 1741 aged 106! That may possibly be a little exageration of his age because there is a Richard Merriman baptised at Brooke on 24 Feb 1638/9 to Thomas and Grace. There is a burial for a Richard Merriman in Brooke in 1639/40 but it doesn't list any parents, it could be this Richard or an earlier one who was in the parish. Richard and Elizabeth did have a son Thomas baptised in 1688 in Pickworth. However, owing to the proximity of Pickworth to the Lincolnshire border, he could also have been born over the county border.
Elizabeth wife of Richard Merryman was buried at Pickworth 3 Dec 1704.
John Merriman son of Edward and Elizabeth was baptised at Pickworth on 20 Jun 1760.
Leicester & Rutland Family History Society have transcribed all Rutland PRs up to 1840 and they are available to purchase on four CDs. As there are Merriman's listed on all of the first 3 CDs which cover until 1812, it is probably worth your while purchasing copies if you do not have access to the original PRs. They are available from here - http://www.lrfhs.org.uk/onlinecdsales.htm
Pickworth PRs are on CD3 and Great Casterton on CD2, however as mentioned there are other Merrimans on CD2 prior to 1812.